The IRS considers a paper tax return filed on the date that the return is deposited in the mail (properly addressed and postage paid). This is the “Mailbox Rule.” Historically, mail was postmarked on the date that the return was dropped in the mailbox, so the date of mailing was easily verified. On November 24, 2025, the U.S. Postal Service finalized a rule clarifying how and when postmarks are applied. Under modern USPS processing, many mail pieces are now postmarked at regional processing facilities, not at the local post office. So now your postmarked date may be later than the date deposited in the mailbox. If the filing date is material, then in order to verify mailing date, certified mail is recommended.