Late in 2025, the U.S. Postal Service changed procedures around its postmarks. Now, mail will not be postmarked until processed by a regional facility, which could be some time after the date actually dropped off at the mailbox or post office.
This could have effects on mail-in ballots, but also directly affects tax filings or payments to tax authorities made via mail. Federal tax law, for many decades, has provided that tax returns, filings, or payments received by the IRS after the due date are nevertheless treated as timely if it was postmarked on or before the due date. This applies to individual and business returns, extension requests, refund claims, payments, and petitions filed with the U.S. Tax Court. The Illinois Department of Revenue also follows these rules.
Thus, a late postmark can cause the filing or payment to be deemed to be late and have real consequences, like denial of refund, penalties, fees etc.
If you choose to mail your tax filing or payment, here are a couple of options for ensuring a timely filing:
Request a manual postmark - post office customers may present a mail piece at a retail counter and request a "manual (local) postmark". This postmark is applied at the time of acceptance, so the date aligns with the date the USPS took possession.
Send the item via registered mail or certified mail. These options have costs, but also provide a receipt that can serve as evidence of timely filing. These must be kept by the taxpayer as the USPS does not keep this information for customers.
It should be noted that the IRS is phasing out accepting tax payments by check. Modern systems offer many opportunities to file and pay various tax filings and payments electronically, including setting up electronic withdrawal from a bank account, or paying via the IRS and state revenue websites (here is the Illinois payment site). The easiest way to not run afoul of deadlines due to these new USPS rules would be to take advantage of those online methods.