feat(account): brand and harden the email pipeline

Swap the net/smtp mailer for go-mail behind the existing Mailer seam: the
Message struct now carries a text + HTML body, TLS mode is chosen from the port
(implicit TLS on 465, else mandatory STARTTLS), a dial timeout bounds the
synchronous send, and no client certificate is needed. Add a branded, image-free,
mobile-friendly ru/en HTML template (with a plain-text alternative) rendering a
large readable code and an ignore-notice footer with a landing link.

Add BACKEND_PUBLIC_BASE_URL config (the canonical origin for the email footer
link, never the request Host — anti-injection), required when a relay is
configured.

Fix the email-login address squat: ProvisionEmail creates the account flagged
is_guest until the code is confirmed, so an abandoned login is reaped like any
guest and its address freed; confirming (login or link) clears the flag. Seed the
new account's language from the client, plumbed through the email-login request.

The confirm deeplink, its transport surface and the send rate-limit land in
follow-up work.

backend/internal/account/account.go

@@ -121,13 +121,30 @@ func (s *Store) ProvisionByIdentity(ctx context.Context, kind, externalID string
 }
 
 // ProvisionEmail returns the account owning the email identity externalID, creating
-// it (unconfirmed) on first contact with browserTZ — the client's detected "±HH:MM"
-// UTC offset — seeded into its time zone. Like ProvisionByIdentity it is race-safe
-// and leaves an existing account untouched, so a returning user's saved zone is never
-// overwritten. The email account is created here (the code-request step), not at the
-// later login, so this is where its zone is seeded.
-func (s *Store) ProvisionEmail(ctx context.Context, externalID, browserTZ string) (Account, error) {
-	return s.provision(ctx, KindEmail, externalID, provisionSeed{timeZone: seedZone(browserTZ)})
+// it on first contact with browserTZ — the client's detected "±HH:MM" UTC offset —
+// seeded into its time zone and language seeded from the client's UI language. Like
+// ProvisionByIdentity it is race-safe and leaves an existing account untouched, so a
+// returning user's saved zone and language are never overwritten. The email account is
+// created here (the code-request step), not at the later login, so this is where its
+// zone and language are seeded. It is created flagged is_guest with an unconfirmed
+// email identity: an abandoned, never-confirmed login is then reaped like any guest,
+// freeing the reserved address, and confirming the code clears the guest flag.
+func (s *Store) ProvisionEmail(ctx context.Context, externalID, browserTZ, language string) (Account, error) {
+	return s.provision(ctx, KindEmail, externalID, provisionSeed{
+		timeZone:          seedZone(browserTZ),
+		preferredLanguage: supportedLanguage(language),
+		isGuest:           true,
+	})
+}
+
+// supportedLanguage returns code normalised to a supported UI language ("en" or
+// "ru"), or "" when it maps to neither, so a new account keeps the 'en' default. It
+// accepts region-tagged codes ("ru-RU").
+func supportedLanguage(code string) string {
+	if lang, _, _ := strings.Cut(strings.ToLower(strings.TrimSpace(code)), "-"); lang == "en" || lang == "ru" {
+		return lang
+	}
+	return ""
 }
 
 // ProvisionRobot provisions (or finds) the durable account backing a robot pool
@@ -239,6 +256,11 @@ type provisionSeed struct {
 	preferredLanguage string
 	displayName       string
 	timeZone          string
+	// isGuest creates the account flagged is_guest. It is set for an email-login
+	// account, which stays a guest until the address is confirmed (so an abandoned,
+	// never-confirmed login is reaped and its address freed); confirming clears the
+	// flag. Platform identities (telegram/vk) are durable from creation.
+	isGuest bool
 }
 
 // seedZone returns browserTZ when it is a well-formed zone to persist at account
@@ -447,8 +469,8 @@ func (s *Store) create(ctx context.Context, kind, externalID string, seed provis
 			tz = "UTC"
 		}
 		insertAccount := table.Accounts.
-			INSERT(table.Accounts.AccountID, table.Accounts.DisplayName, table.Accounts.PreferredLanguage, table.Accounts.TimeZone).
-			VALUES(accountID, seed.displayName, lang, tz).
+			INSERT(table.Accounts.AccountID, table.Accounts.DisplayName, table.Accounts.PreferredLanguage, table.Accounts.TimeZone, table.Accounts.IsGuest).
+			VALUES(accountID, seed.displayName, lang, tz, seed.isGuest).
 			RETURNING(table.Accounts.AllColumns)
 
 		var row model.Accounts

backend/internal/account/email.go

@@ -28,6 +28,15 @@ const (
 	emailCodeMaxAttempts = 5
 )
 
+// Confirmation purposes recorded on a pending confirm-code row. They select what
+// verifying the code or the deeplink token does: sign in (login) or link/confirm the
+// address on the current account (link). Email change and account deletion add
+// further purposes in later stages.
+const (
+	purposeLogin = "login"
+	purposeLink  = "link"
+)
+
 // Errors returned by the email confirm-code flow.
 var (
 	// ErrInvalidEmail is returned for an unparseable email address.
@@ -55,14 +64,47 @@ var (
 // account is refused (ErrEmailTaken) — merging two accounts is the link/merge flow —
 // and using an email as a login reuses this mechanism.
 type EmailService struct {
-	store  *Store
-	mailer Mailer
-	now    func() time.Time
+	store   *Store
+	mailer  Mailer
+	baseURL string
+	now     func() time.Time
 }
 
-// NewEmailService constructs an EmailService over store, sending via mailer.
-func NewEmailService(store *Store, mailer Mailer) *EmailService {
-	return &EmailService{store: store, mailer: mailer, now: func() time.Time { return time.Now().UTC() }}
+// NewEmailService constructs an EmailService over store, sending via mailer. baseURL
+// is the canonical public origin (scheme + host) used to build the one-tap confirm
+// deeplink and the email footer landing link; an empty baseURL omits the deeplink
+// (development / log mailer).
+func NewEmailService(store *Store, mailer Mailer, baseURL string) *EmailService {
+	return &EmailService{store: store, mailer: mailer, baseURL: baseURL, now: func() time.Time { return time.Now().UTC() }}
+}
+
+// issueCode generates a fresh confirm-code for (accountID, email), replaces any prior
+// pending confirmation, and mails the branded code in locale; purpose selects the
+// email wording. Only the SHA-256 hash of the code is stored.
+func (s *EmailService) issueCode(ctx context.Context, accountID uuid.UUID, email, purpose, locale string) error {
+	code, codeHash, err := generateCode()
+	if err != nil {
+		return err
+	}
+	if err := s.store.replacePendingConfirmation(ctx, accountID, email, codeHash, s.now().Add(emailCodeTTL)); err != nil {
+		return err
+	}
+	msg, err := renderConfirmationEmail(purpose, code, "", s.baseURL, locale)
+	if err != nil {
+		return err
+	}
+	msg.To = email
+	return s.mailer.Send(ctx, msg)
+}
+
+// accountLocale returns the account's preferred UI language for localising email,
+// defaulting to "en" when the account cannot be loaded.
+func (s *EmailService) accountLocale(ctx context.Context, accountID uuid.UUID) string {
+	acc, err := s.store.GetByID(ctx, accountID)
+	if err != nil {
+		return "en"
+	}
+	return acc.PreferredLanguage
 }
 
 // RequestCode issues a fresh confirm-code for email to accountID and mails it,
@@ -83,16 +125,7 @@ func (s *EmailService) RequestCode(ctx context.Context, accountID uuid.UUID, ema
 		}
 		return ErrEmailTaken
 	}
-	code, hash, err := generateCode()
-	if err != nil {
-		return err
-	}
-	if err := s.store.replacePendingConfirmation(ctx, accountID, addr, hash, s.now().Add(emailCodeTTL)); err != nil {
-		return err
-	}
-	subject := "Your Scrabble confirmation code"
-	body := fmt.Sprintf("Your confirmation code is %s. It expires in %d minutes.", code, int(emailCodeTTL/time.Minute))
-	return s.mailer.Send(ctx, addr, subject, body)
+	return s.issueCode(ctx, accountID, addr, purposeLink, s.accountLocale(ctx, accountID))
 }
 
 // ConfirmCode verifies code for accountID and email. On success it attaches a
@@ -127,32 +160,23 @@ func (s *EmailService) ConfirmCode(ctx context.Context, accountID uuid.UUID, ema
 }
 
 // RequestLoginCode issues a login confirm-code to the account that owns email,
-// provisioning a fresh (unconfirmed) durable account when the email is new. It is
-// the unauthenticated email-login entry point and, unlike RequestCode,
-// does not refuse an already-confirmed email — that is the ordinary returning-user
-// login. The code is mailed to the address, so only its real owner can complete
-// the login. On first contact browserTZ (the client's detected "±HH:MM" UTC offset)
-// seeds the new account's time zone. It returns the target account id for the
-// subsequent LoginWithCode.
-func (s *EmailService) RequestLoginCode(ctx context.Context, email, browserTZ string) (uuid.UUID, error) {
+// provisioning a fresh (unconfirmed) guest account when the email is new — it becomes
+// durable once the code is confirmed. It is the unauthenticated email-login entry
+// point and, unlike RequestCode, does not refuse an already-confirmed email — that is
+// the ordinary returning-user login. The code is mailed to the address, so only its
+// real owner can complete the login. On first contact browserTZ (the client's
+// detected "±HH:MM" UTC offset) seeds the new account's time zone and language its UI
+// language. It returns the target account id for the subsequent LoginWithCode.
+func (s *EmailService) RequestLoginCode(ctx context.Context, email, browserTZ, language string) (uuid.UUID, error) {
 	addr, err := normalizeEmail(email)
 	if err != nil {
 		return uuid.UUID{}, err
 	}
-	acc, err := s.store.ProvisionEmail(ctx, addr, browserTZ)
+	acc, err := s.store.ProvisionEmail(ctx, addr, browserTZ, language)
 	if err != nil {
 		return uuid.UUID{}, err
 	}
-	code, hash, err := generateCode()
-	if err != nil {
-		return uuid.UUID{}, err
-	}
-	if err := s.store.replacePendingConfirmation(ctx, acc.ID, addr, hash, s.now().Add(emailCodeTTL)); err != nil {
-		return uuid.UUID{}, err
-	}
-	subject := "Your Scrabble login code"
-	body := fmt.Sprintf("Your login code is %s. It expires in %d minutes.", code, int(emailCodeTTL/time.Minute))
-	if err := s.mailer.Send(ctx, addr, subject, body); err != nil {
+	if err := s.issueCode(ctx, acc.ID, addr, purposeLogin, language); err != nil {
 		return uuid.UUID{}, err
 	}
 	return acc.ID, nil
@@ -191,6 +215,9 @@ func (s *EmailService) LoginWithCode(ctx context.Context, email, code string) (A
 	if err := s.store.confirmEmailLogin(ctx, conf.id, acc.ID, addr, s.now()); err != nil {
 		return Account{}, err
 	}
+	if err := s.store.ClearGuest(ctx, acc.ID); err != nil {
+		return Account{}, err
+	}
 	return s.store.GetByID(ctx, acc.ID)
 }
 

backend/internal/account/emailtemplate.go

@@ -0,0 +1,203 @@
+package account
+
+import (
+	"fmt"
+	"html/template"
+	"strings"
+	tmpltext "text/template"
+	"time"
+)
+
+// emailBrandColor is the single accent used in the confirmation email — a calm
+// tile green, matching the "no riot of colours" brief.
+const emailBrandColor = "#2f7d4f"
+
+// confirmEmailView is the fully-localised data the confirmation email templates
+// render. Every string is resolved before rendering, so the templates carry no
+// localisation logic.
+type confirmEmailView struct {
+	Brand        string
+	Heading      string
+	Intro        string
+	Code         string
+	Expiry       string
+	CTALabel     string
+	DeeplinkURL  string
+	FooterIgnore string
+	LandingURL   string
+	LandingLabel string
+	Preheader    string
+	Locale       string
+	Accent       string
+}
+
+// emailCopy is the purpose- and locale-specific wording of a confirmation email.
+type emailCopy struct {
+	Subject      string
+	Preheader    string
+	Heading      string
+	Intro        string
+	CTALabel     string
+	FooterIgnore string
+}
+
+// confirmEmailCopy holds the wording per (purpose, locale). Unknown purposes fall
+// back to the neutral link wording and unknown locales fall back to English.
+var confirmEmailCopy = map[string]map[string]emailCopy{
+	purposeLogin: {
+		"en": {
+			Subject:      "Your Erudit sign-in code",
+			Preheader:    "Your sign-in code",
+			Heading:      "Sign in to Erudit",
+			Intro:        "Enter this code to sign in:",
+			CTALabel:     "Sign in with one tap",
+			FooterIgnore: "If you didn't request this email, you can safely ignore it.",
+		},
+		"ru": {
+			Subject:      "Код для входа в Эрудит",
+			Preheader:    "Ваш код для входа",
+			Heading:      "Вход в Эрудит",
+			Intro:        "Введите этот код, чтобы войти в игру:",
+			CTALabel:     "Войти одним нажатием",
+			FooterIgnore: "Если вы не запрашивали это письмо, просто проигнорируйте его.",
+		},
+	},
+	purposeLink: {
+		"en": {
+			Subject:      "Your Erudit confirmation code",
+			Preheader:    "Your confirmation code",
+			Heading:      "Confirm your e-mail",
+			Intro:        "Enter this code to confirm your address:",
+			CTALabel:     "Confirm with one tap",
+			FooterIgnore: "If you didn't request this email, you can safely ignore it.",
+		},
+		"ru": {
+			Subject:      "Код подтверждения Эрудит",
+			Preheader:    "Ваш код подтверждения",
+			Heading:      "Подтверждение e-mail",
+			Intro:        "Введите этот код, чтобы подтвердить адрес:",
+			CTALabel:     "Подтвердить одним нажатием",
+			FooterIgnore: "Если вы не запрашивали это письмо, просто проигнорируйте его.",
+		},
+	},
+}
+
+// emailBrand is the brand wordmark per locale.
+var emailBrand = map[string]string{"en": "Erudit", "ru": "Эрудит"}
+
+// emailExpiry formats the code-lifetime line per locale (abbreviated minutes to
+// avoid plural agreement).
+func emailExpiry(locale string, d time.Duration) string {
+	min := int(d / time.Minute)
+	if locale == "ru" {
+		return fmt.Sprintf("Код действует %d мин.", min)
+	}
+	return fmt.Sprintf("The code is valid for %d minutes.", min)
+}
+
+// normalizeLocale maps an account language to a supported email locale, defaulting
+// to English.
+func normalizeLocale(locale string) string {
+	if locale == "ru" {
+		return "ru"
+	}
+	return "en"
+}
+
+// renderConfirmationEmail builds the branded confirmation email for purpose in
+// locale: a large readable code plus a one-tap deeplink button, with an
+// ignore-notice footer and a landing link. Both a plain-text body and an HTML
+// alternative are produced. deeplinkURL is the absolute /confirm link and
+// landingURL the public landing origin.
+func renderConfirmationEmail(purpose, code, deeplinkURL, landingURL, locale string) (Message, error) {
+	loc := normalizeLocale(locale)
+	byLocale, ok := confirmEmailCopy[purpose]
+	if !ok {
+		byLocale = confirmEmailCopy[purposeLink]
+	}
+	cp := byLocale[loc]
+	view := confirmEmailView{
+		Brand:        emailBrand[loc],
+		Heading:      cp.Heading,
+		Intro:        cp.Intro,
+		Code:         code,
+		Expiry:       emailExpiry(loc, emailCodeTTL),
+		CTALabel:     cp.CTALabel,
+		DeeplinkURL:  deeplinkURL,
+		FooterIgnore: cp.FooterIgnore,
+		LandingURL:   landingURL,
+		LandingLabel: emailBrand[loc],
+		Preheader:    cp.Preheader,
+		Locale:       loc,
+		Accent:       emailBrandColor,
+	}
+	var html strings.Builder
+	if err := confirmEmailHTML.Execute(&html, view); err != nil {
+		return Message{}, fmt.Errorf("account: render confirmation email (html): %w", err)
+	}
+	var text strings.Builder
+	if err := confirmEmailText.Execute(&text, view); err != nil {
+		return Message{}, fmt.Errorf("account: render confirmation email (text): %w", err)
+	}
+	return Message{Subject: cp.Subject, Text: text.String(), HTML: html.String()}, nil
+}
+
+// confirmEmailHTML is a compact, image-free, mobile-friendly HTML email. Layout is
+// table-based for broad mail-client compatibility and all styling is inlined
+// because clients strip <style> blocks.
+var confirmEmailHTML = template.Must(template.New("confirmEmailHTML").Parse(`<!DOCTYPE html>
+<html lang="{{.Locale}}">
+<head>
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+<title>{{.Brand}}</title>
+</head>
+<body style="margin:0;padding:0;background:#f4f5f7;">
+<span style="display:none;max-height:0;overflow:hidden;opacity:0;">{{.Preheader}}</span>
+<table role="presentation" width="100%" cellpadding="0" cellspacing="0" style="background:#f4f5f7;padding:24px 12px;">
+<tr><td align="center">
+<table role="presentation" width="100%" cellpadding="0" cellspacing="0" style="max-width:460px;background:#ffffff;border:1px solid #e5e7eb;border-radius:14px;overflow:hidden;font-family:-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;">
+<tr><td style="padding:28px 32px 4px;">
+<div style="font-size:15px;font-weight:700;letter-spacing:.04em;color:{{.Accent}};">{{.Brand}}</div>
+</td></tr>
+<tr><td style="padding:8px 32px 0;">
+<h1 style="margin:0;font-size:20px;line-height:1.3;color:#111827;font-weight:600;">{{.Heading}}</h1>
+<p style="margin:12px 0 0;font-size:15px;line-height:1.5;color:#374151;">{{.Intro}}</p>
+</td></tr>
+<tr><td style="padding:18px 32px 0;">
+<div style="font-size:34px;font-weight:700;letter-spacing:8px;text-align:center;color:#111827;background:#f3f4f6;border-radius:10px;padding:18px 0;font-family:'SFMono-Regular',Consolas,Menlo,monospace;">{{.Code}}</div>
+<p style="margin:10px 0 0;font-size:13px;line-height:1.5;color:#6b7280;text-align:center;">{{.Expiry}}</p>
+</td></tr>
+{{if .DeeplinkURL}}<tr><td style="padding:22px 32px 0;" align="center">
+<a href="{{.DeeplinkURL}}" style="display:inline-block;background:{{.Accent}};color:#ffffff;text-decoration:none;font-size:15px;font-weight:600;padding:12px 26px;border-radius:9px;">{{.CTALabel}}</a>
+</td></tr>
+{{end}}<tr><td style="padding:26px 32px 28px;">
+<hr style="border:none;border-top:1px solid #eceef1;margin:0 0 16px;">
+<p style="margin:0;font-size:12px;line-height:1.6;color:#9ca3af;">{{.FooterIgnore}}</p>
+<p style="margin:10px 0 0;font-size:12px;color:#9ca3af;"><a href="{{.LandingURL}}" style="color:#6b7280;text-decoration:none;">{{.LandingLabel}}</a></p>
+</td></tr>
+</table>
+</td></tr>
+</table>
+</body>
+</html>
+`))
+
+// confirmEmailText is the plain-text alternative (and multipart fallback).
+var confirmEmailText = tmpltext.Must(tmpltext.New("confirmEmailText").Parse(`{{.Brand}}
+
+{{.Heading}}
+
+{{.Intro}}
+
+    {{.Code}}
+
+{{.Expiry}}
+
+{{if .DeeplinkURL}}{{.CTALabel}}:
+{{.DeeplinkURL}}
+
+{{end}}—
+{{.FooterIgnore}}
+{{.LandingLabel}} — {{.LandingURL}}
+`))

backend/internal/account/emailtemplate_test.go

@@ -0,0 +1,52 @@
+package account
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestRenderConfirmationEmail checks that each (purpose, locale) renders a localised
+// subject, embeds the code and the one-tap deeplink in both bodies, and produces HTML.
+func TestRenderConfirmationEmail(t *testing.T) {
+	const deeplink = "https://erudit-game.ru/app/#/confirm/tok123"
+	cases := []struct {
+		name, purpose, locale, subjectSub string
+	}{
+		{"login ru", purposeLogin, "ru", "вход"},
+		{"login en", purposeLogin, "en", "sign-in"},
+		{"link ru", purposeLink, "ru", "подтвержд"},
+		{"link en", purposeLink, "en", "confirmation"},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			msg, err := renderConfirmationEmail(c.purpose, "123456", deeplink, "https://erudit-game.ru", c.locale)
+			if err != nil {
+				t.Fatalf("render: %v", err)
+			}
+			if !strings.Contains(strings.ToLower(msg.Subject), c.subjectSub) {
+				t.Errorf("subject %q does not contain %q", msg.Subject, c.subjectSub)
+			}
+			if !strings.Contains(msg.Text, "123456") || !strings.Contains(msg.HTML, "123456") {
+				t.Error("code missing from a body")
+			}
+			if !strings.Contains(msg.Text, deeplink) || !strings.Contains(msg.HTML, "confirm/tok123") {
+				t.Error("deeplink missing from a body")
+			}
+			if !strings.Contains(msg.HTML, "<html") {
+				t.Error("HTML body is not HTML")
+			}
+		})
+	}
+}
+
+// TestRenderConfirmationEmailUnknownLocaleDefaultsEnglish falls back to English for an
+// unsupported locale rather than erroring or emitting an empty subject.
+func TestRenderConfirmationEmailUnknownLocaleDefaultsEnglish(t *testing.T) {
+	msg, err := renderConfirmationEmail(purposeLogin, "000000", "", "https://erudit-game.ru", "de")
+	if err != nil {
+		t.Fatalf("render: %v", err)
+	}
+	if !strings.Contains(strings.ToLower(msg.Subject), "sign-in") {
+		t.Errorf("unknown locale should default to English, got subject %q", msg.Subject)
+	}
+}

backend/internal/account/link.go

@@ -26,16 +26,7 @@ func (s *EmailService) RequestLinkCode(ctx context.Context, accountID uuid.UUID,
 	if err != nil {
 		return err
 	}
-	code, hash, err := generateCode()
-	if err != nil {
-		return err
-	}
-	if err := s.store.replacePendingConfirmation(ctx, accountID, addr, hash, s.now().Add(emailCodeTTL)); err != nil {
-		return err
-	}
-	subject := "Your Scrabble confirmation code"
-	body := fmt.Sprintf("Your confirmation code is %s. It expires in %d minutes.", code, int(emailCodeTTL/time.Minute))
-	return s.mailer.Send(ctx, addr, subject, body)
+	return s.issueCode(ctx, accountID, addr, purposeLink, s.accountLocale(ctx, accountID))
 }
 
 // ConfirmLink verifies code for (accountID, email) and reports the address's

backend/internal/account/mailer.go

@@ -3,23 +3,37 @@ package account
 import (
 	"context"
 	"fmt"
-	"net"
-	"net/smtp"
+	"strconv"
+	"time"
 
+	"github.com/wneessen/go-mail"
 	"go.uber.org/zap"
 )
 
+// Message is a transactional email to send through a Mailer. Text is the
+// required plain-text body and doubles as the multipart/alternative fallback;
+// HTML, when non-empty, is the preferred body a capable client renders instead.
+type Message struct {
+	To      string
+	Subject string
+	Text    string
+	HTML    string
+}
+
 // Mailer delivers a transactional email. It is the seam behind which the email
 // confirm-code flow sends codes, so the relay is swappable and unit tests use a
-// fixture (see docs/TESTING.md: no real network in tests). The context is offered
-// for cancellation; the standard-library SMTP implementation sends synchronously
-// and ignores it.
+// fixture (see docs/TESTING.md: no real network in tests). The context bounds the
+// delivery and is honoured by the SMTP implementation.
 type Mailer interface {
-	Send(ctx context.Context, to, subject, body string) error
+	Send(ctx context.Context, msg Message) error
 }
 
 // SMTPConfig configures the SMTP relay. An empty Host selects the LogMailer
 // instead, so a deployment without a relay still runs (the code lands in the log).
+// TLS is always used and no client certificate is required — only the server
+// certificate is validated against the system roots. The Port selects the TLS
+// mode: smtpImplicitTLSPort (465) uses implicit TLS (SSL); any other port uses
+// mandatory STARTTLS.
 type SMTPConfig struct {
 	Host     string
 	Port     string
@@ -28,8 +42,19 @@ type SMTPConfig struct {
 	From     string
 }
 
-// SMTPMailer sends mail through an SMTP relay using the standard library. When a
-// username is set it authenticates with PLAIN; otherwise it relays unauthenticated.
+const (
+	// smtpImplicitTLSPort is the port on which the relay is dialled with implicit
+	// TLS (SSL). Any other port negotiates mandatory STARTTLS instead.
+	smtpImplicitTLSPort = 465
+	// smtpDialTimeout bounds a single relay connect-and-send. The confirm-code send
+	// is synchronous on the request path, so an unreachable relay must fail fast
+	// rather than hold the request open.
+	smtpDialTimeout = 15 * time.Second
+)
+
+// SMTPMailer sends mail through an SMTP relay using go-mail. When a username is
+// set it authenticates, auto-discovering the strongest mechanism the relay
+// advertises; otherwise it relays unauthenticated.
 type SMTPMailer struct {
 	cfg SMTPConfig
 }
@@ -39,29 +64,49 @@ func NewSMTPMailer(cfg SMTPConfig) SMTPMailer {
 	return SMTPMailer{cfg: cfg}
 }
 
-// Send delivers a plain-text UTF-8 message to to via the configured relay.
-func (m SMTPMailer) Send(_ context.Context, to, subject, body string) error {
-	addr := net.JoinHostPort(m.cfg.Host, m.cfg.Port)
-	var auth smtp.Auth
-	if m.cfg.Username != "" {
-		auth = smtp.PlainAuth("", m.cfg.Username, m.cfg.Password, m.cfg.Host)
+// Send delivers a UTF-8 message to msg.To via the configured relay. When msg.HTML
+// is set the message is multipart/alternative (plain text plus HTML); otherwise
+// it is plain text only.
+func (m SMTPMailer) Send(ctx context.Context, msg Message) error {
+	port, err := strconv.Atoi(m.cfg.Port)
+	if err != nil {
+		return fmt.Errorf("account: invalid SMTP port %q: %w", m.cfg.Port, err)
 	}
-	if err := smtp.SendMail(addr, auth, m.cfg.From, []string{to}, message(m.cfg.From, to, subject, body)); err != nil {
-		return fmt.Errorf("account: send mail to %s: %w", to, err)
+	opts := []mail.Option{mail.WithTimeout(smtpDialTimeout)}
+	if port == smtpImplicitTLSPort {
+		opts = append(opts, mail.WithSSLPort(false))
+	} else {
+		opts = append(opts, mail.WithPort(port), mail.WithTLSPortPolicy(mail.TLSMandatory))
+	}
+	if m.cfg.Username != "" {
+		opts = append(opts,
+			mail.WithSMTPAuth(mail.SMTPAuthAutoDiscover),
+			mail.WithUsername(m.cfg.Username),
+			mail.WithPassword(m.cfg.Password),
+		)
+	}
+	client, err := mail.NewClient(m.cfg.Host, opts...)
+	if err != nil {
+		return fmt.Errorf("account: build mail client: %w", err)
+	}
+	out := mail.NewMsg()
+	if err := out.From(m.cfg.From); err != nil {
+		return fmt.Errorf("account: set From %q: %w", m.cfg.From, err)
+	}
+	if err := out.To(msg.To); err != nil {
+		return fmt.Errorf("account: set To %q: %w", msg.To, err)
+	}
+	out.Subject(msg.Subject)
+	out.SetBodyString(mail.TypeTextPlain, msg.Text)
+	if msg.HTML != "" {
+		out.AddAlternativeString(mail.TypeTextHTML, msg.HTML)
+	}
+	if err := client.DialAndSendWithContext(ctx, out); err != nil {
+		return fmt.Errorf("account: send mail to %s: %w", msg.To, err)
 	}
 	return nil
 }
 
-// message renders a minimal RFC 5322 plain-text email.
-func message(from, to, subject, body string) []byte {
-	return []byte("From: " + from + "\r\n" +
-		"To: " + to + "\r\n" +
-		"Subject: " + subject + "\r\n" +
-		"MIME-Version: 1.0\r\n" +
-		"Content-Type: text/plain; charset=UTF-8\r\n" +
-		"\r\n" + body + "\r\n")
-}
-
 // LogMailer logs the message instead of sending it. It is the default when no
 // SMTP relay is configured and is intended for development only: it logs the body,
 // which carries the confirm-code, so it must not be used in production.
@@ -74,11 +119,12 @@ func NewLogMailer(log *zap.Logger) LogMailer {
 	return LogMailer{log: log}
 }
 
-// Send logs the message at info level and reports success.
-func (m LogMailer) Send(_ context.Context, to, subject, body string) error {
+// Send logs the message at info level and reports success. It logs the plain-text
+// body only (which carries the confirm-code); the HTML alternative is omitted.
+func (m LogMailer) Send(_ context.Context, msg Message) error {
 	if m.log != nil {
 		m.log.Info("email not sent (log mailer)",
-			zap.String("to", to), zap.String("subject", subject), zap.String("body", body))
+			zap.String("to", msg.To), zap.String("subject", msg.Subject), zap.String("body", msg.Text))
 	}
 	return nil
 }