SMTP

SMTP is a second front door onto POST /v1/send. Apps that only have host / port / user / password fields (Twenty, WordPress, Laravel, PHPMailer) can send through Letter without calling HTTP. The mail uses the same project API key, the same verified sending domain, the same suppression and plan gates, and lands in the same transactional log.

If you can use an SDK or POST /v1/send, do that. SMTP exists for software you do not control.

Prerequisites

A project API key (lt_live_…) from Settings → Project tokens
A verified sending domain (Letter subdomain or your own)

Settings


Host	`smtp.letter.app`
Port	`465` (implicit TLS). Alternate: `2465` if 465 is blocked.
Username	`letter`
Password	your project API key
Encryption	SSL / TLS

There is no separate SMTP user. Identity is the API key.

Port 587 (STARTTLS) is not on v1. Twenty, WordPress, and Laravel all accept 465 with SSL.

Quick start

Point the app at the table above. The From address must be on this project’s verified sending domain (the same From you configured under Settings → Domain).

Twenty

In Settings → Admin Panel → EMAIL_SETTINGS:

EMAIL_DRIVER=smtp
EMAIL_SMTP_HOST=smtp.letter.app
EMAIL_SMTP_PORT=465
EMAIL_SMTP_USER=letter
EMAIL_SMTP_PASSWORD=lt_live_…
EMAIL_SMTP_NO_TLS=false
EMAIL_FROM_ADDRESS=hello@your-verified-domain
EMAIL_FROM_NAME=You

WordPress (WP Mail SMTP)

Mailer: Other SMTP. Host smtp.letter.app, encryption SSL, port 465, Auto TLS on, authentication on, username letter, password the API key.

Laravel

MAIL_MAILER=smtp
MAIL_HOST=smtp.letter.app
MAIL_PORT=465
MAIL_USERNAME=letter
MAIL_PASSWORD=lt_live_…
MAIL_ENCRYPTION=ssl
MAIL_FROM_ADDRESS=hello@your-verified-domain
MAIL_FROM_NAME="${APP_NAME}"

What happens on send

AUTH PLAIN/LOGIN — username must be letter; password is the project key.
One RCPT TO. A second recipient is refused (552).
The MIME body is flattened to to, from, subject, html / text, and optional headers, then posted to /v1/send with tag: smtp.
The To address is upserted as a contact (source: "api"). An existing contact is reused. A new address when the workspace is at its contact cap is refused (554, same as 403 contact_quota_exceeded on HTTP).
The monthly email cap, daily shared-domain cap, suppressions, and suspension gates are the HTTP ones.

Attachments are refused (552), not dropped. v1 is SES Simple: one recipient, html/text, no Cc/Bcc. Bodies are capped at 500 KB.

Idempotency

Set X-Letter-Idempotency-Key or Letter-Idempotency-Key (8–128 characters) to make client retries safe. If neither is present, a Message-ID in that length range is used.

Custom headers

Passed through to /v1/send (max 20). Envelope headers and List-Unsubscribe* are reserved — same list as the HTTP route.

Errors

SMTP reply codes are the HTTP errors in envelope form. Look up the send in the dashboard; there are no SMTP server logs.

SMTP	HTTP	When
`535`	`401`	Bad username or API key
`550`	`403` `suppressed`	Recipient bounced, complained, or was admin-suppressed
`554`	`403` / `400`	Contact cap, email quota, daily cap, suspended, billing, bad From, validation
`552`	—	Second recipient, attachment, or oversized body
`421`	`429`	Rate limited. Retry later
`451`	`5xx`	Temporary failure. Retry with the same idempotency key

403 reasons from the API (contact_quota_exceeded, email_quota_exceeded, daily_cap, billing_required, account_suspended, suppressed) apply unchanged.

FAQ

Where do I see the email? The project’s Transactional page, or GET /v1/projects/{slug}/messages / letter messages list. SMTP sends are tagged smtp.

Do rate limits apply? Yes — the same per-key limit as /v1/send.

Can I use SMTP from Rails? Prefer the Ruby SDK ActionMailer :letter delivery method. SMTP is for apps you cannot change.

Do you offer IMAP or inbound SMTP? No.