Resend Email

Resend Email Integration

Resend is a transactional email service. It supports both SMTP and a REST API. Use SMTP for simple sends and the HTTP API for scheduled emails, cancellation, and listing.

API keys

Resend has scoped API keys. This matters:

Best practice: use a send-only key for SMTP/sending and a separate full-access key for admin operations (cancel, list). Never use the full-access key for routine sends.

SMTP_PASS=re_eiuw...        # send-only, used for SMTP and scheduled sends
RESEND_SUDO_KEY=re_Ak8U...  # full-access, used for cancel/list operations

Domain verification

Before sending, your domain must be verified in Resend. Add the DNS records (TXT, CNAME) from the Resend dashboard. The from address must use a verified domain.

If you don't have your own domain verified, you can use a domain from another project that's already set up — just make sure the SMTP_FROM address uses that domain.

Sending via SMTP (Go)

Standard net/smtp works. The SMTP password is the Resend API key.

auth := smtp.PlainAuth("", "resend", apiKey, "smtp.resend.com")
addr := "smtp.resend.com:587"
smtp.SendMail(addr, auth, from, []string{to}, []byte(msg))

Config:

SMTP_HOST=smtp.resend.com
SMTP_PORT=587
SMTP_USER=resend
SMTP_PASS=re_your_api_key

Sending via HTTP API

curl -X POST https://api.resend.com/emails \
  -H "Authorization: Bearer re_your_api_key" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "App <noreply@yourdomain.com>",
    "to": ["user@example.com"],
    "subject": "Hello",
    "text": "Plain text body"
  }'

Response: {"id": "49a3999c-0ce1-4ea6-ab68-afcd6dc2e794"}

Scheduled emails

Use the scheduled_at parameter in the HTTP API (not available via SMTP):

payload := map[string]any{
    "from":         "App <noreply@yourdomain.com>",
    "to":           []string{"user@example.com"},
    "subject":      "Reminder",
    "text":         "Your reminder body",
    "scheduled_at": sendAt.UTC().Format(time.RFC3339),
}

scheduled_at accepts:

• ISO 8601 / RFC3339: "2026-05-16T00:18:20Z"
• Natural language: "in 1 min", "in 30 days"

Important: Save the returned email ID — you'll need it to cancel. Log it and/or return it in your API response.

Cancelling scheduled emails

Requires a full-access API key (send-only keys return 401):

curl -X POST https://api.resend.com/emails/{id}/cancel \
  -H "Authorization: Bearer re_full_access_key" \
  -H "Content-Type: application/json"

In Go:

req, _ := http.NewRequest("POST", "https://api.resend.com/emails/"+emailID+"/cancel", nil)
req.Header.Set("Authorization", "Bearer "+sudoKey)

Listing emails

Also requires full-access key:

curl -X GET 'https://api.resend.com/emails?limit=100' \
  -H "Authorization: Bearer re_full_access_key"

Query params:

• limit (1-100, default 20)
• after / before (email ID for pagination)

Response includes id, to, from, subject, scheduled_at, last_event, created_at.

Filter for scheduled emails:

curl -s -H "Authorization: Bearer $KEY" \
  "https://api.resend.com/emails?limit=100" | \
  jq '.data[] | select(.scheduled_at != null)'

Go integration pattern

Use an interface so you can swap between console (dev) and real (prod) senders:

type EmailSender interface {
    Send(to, subject, body string) error
    SendScheduled(to, subject, body string, sendAt time.Time) (emailID string, err error)
    CancelScheduled(emailID string) error
}

• ConsoleSender: prints to stdout in dev, returns fake IDs
• SMTPSender: uses SMTP for Send, Resend HTTP API for SendScheduled/CancelScheduled
• Switch based on whether SMTP_HOST is set

The SMTPSender needs two keys:

type SMTPSender struct {
    Host    string
    Port    string
    User    string
    Pass    string // send-only key, doubles as SMTP password
    From    string
    SudoKey string // full-access key for cancel/list
}

Gotchas

• Send-only vs full-access keys: The SMTP password/send key cannot cancel or list emails. You get a 401 with "This API key is restricted to only send emails". Use a separate full-access key.
• Domain verification required: Sending from an unverified domain returns 550 The domain is not verified.
• SMTP user is always "resend": Not your email, not your API key — literally the string resend.
• Scheduled emails can't be modified: You can only cancel and re-create.
• Max 50 recipients per email: Use multiple sends for bulk.