What is pm.mtasv.net?
pm.mtasv.net is the hostname used by Postmark for their bounce tracking. It shows up in the headers of an email as
What is a "Return Path"?
When an email server receives an email that it cannot handle for whatever reason, it needs to be report that back to the sender. The
Return-Path email address is where the email server sends that error report.
Return-Path address is also commonly known as a bounce address because it is used for "bounces"; ie. emails that cannot be delivered and is bounced back to the sender.
Having a custom Return-Path address that matches your domain name is important in order to achieve SPF-alignment.