For your provider

Postmark

Postmark says it is no longer required to include Postmark in your own SPF record. Mail sent through Postmark passes SPF on Postmark's Return-Path by default.

CNAME → pm.mtasv.net

A custom Return-Path is a CNAME that points at pm.mtasv.net. Postmark says that CNAME already carries SPF, and that you cannot put a TXT SPF record on the same name as that CNAME. Adding include:spf.mtasv.net on the root domain is optional, and Postmark says it does not replace the CNAME. For DMARC to pass on SPF, Postmark says the policy has to stay aspf=r, because the Return-Path is a subdomain.