USPS Postmark Changes in 2026: Could postmark delays put your tax returns, ballots, and bills at risk this year? USPS is applying most postmarks at regional processing centers, not local post offices. Fewer daily pickups and longer transport routes mean mail can sit longer before being processed. As a result, a letter mailed today may receive tomorrow’s date—or later—on its postmark. This shift is part of USPS’s long-term modernization plan.