Product

SMTP relay

Port 587 · STARTTLS.

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SMTP
Host:     smtp.srs-postline.com
Port:     587
Security: STARTTLS
Username: <project SMTP username>
Password: <project SMTP password>

Production checklist

  • Verify ownership of the From domain.
  • Publish the supplied DKIM record and authorize the return-path infrastructure through SPF.
  • Production API keys are available immediately after signup.
  • Before domain verification, use sandbox@srs-postline.com only for a project-member recipient.
  • Configure DMARC alignment and review the result before sending to external recipients.

Authentication terminology

Postline DKIM-signs messages. SPF is an authorization policy published in DNS. DMARC evaluates alignment between the visible From domain and an authenticated SPF or DKIM identity; SPF and DMARC are not applied as message signatures.

Delivery outcomes

A recipient server accepting a message does not prove inbox placement. Treat accepted, deferred, hard bounce, soft bounce, complaint, suppressed, spam placement, and inbox placement as different outcomes.

Current service limits

The REST API rejects attachments. It accepts only In-Reply-To, References, List-Unsubscribe and List-Unsubscribe-Post custom headers (up to 8). Hosted mail is separate: the web client supports up to 20 attachments, 25 MiB total; the released desktop client has a 10 MiB sending limit. HTTP 202 and email.accepted mean acceptance into the processing pipeline; SMTP acceptance does not guarantee delivery or inbox placement.

Resources

Start safely with PostlineTransactional email REST APICurrent availability