How to send from subdomains?

To send a fully authenticated email, each subdomain you want to send from will have to be added to Postmark individually.

DKIM and Return-Path are domain-specific, so each subdomain you add will have a unique DKIM and Return-Path records for Postmark. The separate DKIM and Return-Path records for your subdomains is the part that makes them unique so that their sending reputation is separate.

Sending from a subdomain would require you to verify the subdomain with DKIM and a custom Return-Path.