The Illinois Department of Revenue (IDOR) is warning taxpayers that recent U.S. Postal Service (USPS) changes to postmarks could affect 2026 tax filings.
Under the new system, postmarks will show when mail is first processed, not when it is dropped off, which could create potential risks for meeting deadlines on time-sensitive items like tax returns and payments.
IDOR urges taxpayers to mail documents earlier than usual, ahead of the April 15 deadline, to avoid penalties. Those who are mailing close to the deadline should request a free manual postmark at the post office, which will reflect the actual drop-off date. IDOR also emphasizes that electronic filing is the most reliable option to ensure timely submission. Alternatively, taxpayers can drop off returns at an IDOR office, where documents will be date-stamped as proof of receipt.
For more information on this change, click here.