# SMTP Connect an SMTP client using your existing API key. ## Connection settings Host smtp.usesendy.com Port 2525 Encryption STARTTLS Username apikey Password Your full API key Authentication PLAIN or LOGIN after STARTTLS Use the literal username `apikey` and the full key, including `snd_key_`, as the password. Connect with SMTP and upgrade using STARTTLS before authentication. Implicit TLS on port 465 is not supported. For application setup, see [Nodemailer](https://usesendy.com/docs/integrations/nodemailer), [Laravel](https://usesendy.com/docs/integrations/laravel), [Django](https://usesendy.com/docs/integrations/django), [Rails](https://usesendy.com/docs/integrations/rails), [PHPMailer](https://usesendy.com/docs/integrations/phpmailer), or [WordPress](https://usesendy.com/docs/integrations/wordpress). Want to keep using `sendmail` on your VPS? Configure [Exim4](https://usesendy.com/docs/smtp/exim4) or [Postfix](https://usesendy.com/docs/smtp/postfix) to relay outbound email through Sendy. Your applications and scripts can keep their local mail submission setup, with Sendy’s SMTP credentials managed by the mail server. Sender verification still applies. ## Send with curl Save the following as `email.eml`. Preserve the blank line between headers and body: email.eml ``` From: My App To: recipient@example.com Subject: Welcome MIME-Version: 1.0 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

Welcome

Your account is ready.

``` SMTP submission · curl ``` curl --url 'smtp://smtp.usesendy.com:2525' --ssl-reqd \ --user 'apikey:API_KEY' \ --mail-from 'noreply@example.com' \ --mail-rcpt 'recipient@example.com' \ --upload-file email.eml ``` Replace `API_KEY`, the sender, and the recipient in both the file and command. Your account password is not used. ## Message requirements * Use verified domains on your account for the MIME `From` header and any nonempty SMTP `MAIL FROM`. Delivery-failure notifications can use `MAIL FROM:<>`; the MIME `From` domain must still be verified. Display names belong in the MIME header. * The optional `MAIL FROM` parameter `AUTH=` accepts an xtext-encoded submitter address or `<>` for an unknown submitter. Your API key determines account access and sending permissions; `AUTH=` does not change them. * Recipients come from the SMTP envelope. `To` and `Cc` headers alone do not add recipients. * MIME preserves HTML, plain-text alternatives, attachments, and reply-to headers. Use your mail library to encode non-ASCII headers. Email addresses must be ASCII. * Provide one `From` header and a nonblank subject of up to 255 bytes. `Sender` and `Resent-*` headers are unsupported. Recipients per message Up to 50 distinct recipients. Message size 1 MiB by default, including encoded attachments. Line length Up to 1,000 bytes, including its line ending. Configured size limit Advertised in the SMTP `SIZE` capability. ## Retry safely with an idempotency key Add one optional email header and reuse its value when retrying the same email: Email header ``` Sendy-Idempotency-Key: welcome/user-123 ``` Keys contain 1 to 128 printable ASCII bytes without spaces. They are case-sensitive and scoped to your account across its API keys, separately from HTTP idempotency keys. Matching retries return `250` with the original job IDs, without creating jobs or reserving credit again. Keys remain reserved while the original job records are retained; there is no time-based expiry. Without this header, each submission creates new jobs. Keep the envelope sender, recipient set, and cleaned MIME message unchanged. Recipient order and generated `Date`, `Message-ID`, and `Received` headers do not affect matching. Other headers and MIME body bytes do, including multipart boundaries and attachment encoding. Preserve the composed raw message for multipart retries. Reusing a key for a different email returns `554 5.5.0`. Use a new key for new content or recipients. Empty, repeated, or malformed key headers return `554`. Sendy removes the key header before delivery. ## Queued response SMTP response ``` 250 2.0.0 Queued jobs 123,124 ``` Sendy creates one job per recipient. This response confirms storage, not inbox delivery. Look up each returned ID with [Get email status](https://usesendy.com/docs/api/email-status) or open [Emails](https://usesendy.com/emails). Without an idempotency key, resubmitting after a lost response can create duplicate jobs. See [Errors](https://usesendy.com/docs/errors) for retry guidance.