Production checklist
Verify ownership of the From domain. Publish the supplied DKIM record and authorize the return-path infrastructure through SPF.
SMTP relay
SMTP relay
Port 587 · STARTTLS.
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.
Host: smtp.srs-postline.com
Port: 587
Security: STARTTLS
Username: <project SMTP username>
Password: <project SMTP password>