The USPS postmark is now when it is processed, not received. This absolutely impacts mailing tax returns and checks on time.

A better idea is to not paperfile tax returns (unless they have to be - some do), and to pay online at the IRS website.

https://www.aarp.org/government-elections/usps-postmark-changes/?msockid=01c16f4b41b5643533037ba1402765a4

USPS Postmark Rules Change: What You Need to Know

Dates on mail now reflect when it’s processed, not when it’s dropped off

AARP