SMTP relay

Drop-in SMTP on port 587 with STARTTLS for any language or mail client.

Production checklist

Verify ownership of the From domain. Publish the supplied DKIM record and authorize the return-path infrastructure through SPF.

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 access model

Current dashboard keys use srs_live_. Before a customer From-domain is verified, use the shared sandbox sender only for project-member recipients. After DNS verification, external sending is governed by plan quotas, suppression, recipient throttles, and abuse controls. A 202 response means queued, not delivered.

SMTP

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

SMTP relay

Resources