The United States Postal Service has announced that, as of Dec. 24, postmarks can no longer be considered proof of mailing on a certain date, thanks to changes in the way mail is collected and processed.
Because the postal service has been closing local processing centers, mail may not be postmarked until it arrives at larger regional processing centers, creating a gap between when you drop off your correspondence off at a post office, and when the USPS officially recognizes receiving it — creating problems for time-sensitive tax filings.
To avoid potential problems with notices, correspondence, payments, and other paper-filed returns, you should request a hand-stamped postmark at the USPS counter to ensure the actual mailing date is recorded; buy postage at the counter, where printed labels include the correct acceptance date; or use certified or registered mail, which provides evidence of the mailing date.