For decades, lawyers have relied on the assumption that if a document was placed in the mail by the deadline, the postmark would confirm it. The long-standing “mailbox rule” made the postmark a quiet but dependable form of proof that something was mailed on time.

That assumption now deserves a second look.

In late December 2025, the United States Postal Service updated its guidance on postmarking practices. Under the revised approach, mail may be postmarked when it is processed at a regional facility, rather than when it is dropped in a local mailbox or post office. As a result, a document mailed on time can still receive a postmark dated after the applicable deadline, depending on processing schedules.

This change matters because many legal and business deadlines still hinge on a “postmarked by” standard. Even a short delay in processing can create unnecessary disputes over timeliness, despite a good-faith effort to mail documents on time.

We most often see postmark-dependent deadlines arise in areas such as:

• Tax filings and Tax Court petitions
  
• Certain federal administrative law and employment law filings
  
• Certain federal applications
  
• Contractual notice provisions

In these contexts, the underlying filing may be substantively correct, but a delayed postmark can raise questions that are avoidable with a few practical adjustments. If your practice involves one of these areas, be sure to check whether the postmark date determines when a document has been timely filed.

When you are dealing with a postmark deadline, you should rely on certain procedures to protect yourself and your clients. Options include requesting a manual postmark at the post office counter, using certified or registered mail, relying on carriers that provide dated receipts, or turning to electronic filing where permitted.

The takeaway is not alarm, but awareness. A postmark is no longer the automatic safety net it once was and a small change in mailing habits can help prevent a much larger headache later.