Postmark

Postmark documents CIDR ranges used for outbound SMTP and related traffic so you can allowlist them on firewalls. Prefer domain-based rules when possible — ranges can change.

Postmark IPs for firewalls

Documentation

Official CIDR ranges documented for Postmark SMTP/API firewall allowlists.

https://postmarkapp.com/support/article/800-ips-for-firewalls

Published prefixes

50.31.156.96/27
50.31.205.0/24
50.31.205.204/30
104.245.209.192/26
147.75.195.176/30
147.75.202.84/30
147.75.205.84/30