Internal Revenue Service
Federal Tax Return Form 1040 WITHOUT Payment Enclosed:
Internal Revenue Service
Austin, TX 73301-0002
Federal Estimated Tax Vouchers with Payment:
Internal Revenue Service
Mailbox Rule:
A return or payment postmarked on or before a due date is considered filed or paid on the due date even if received by the IRS after the due date (IRC 6501 & 6513)
An original return claiming a refund or credit that is postmarked before the end of the three-year limitation period is considered filed on the postmark date (REG 301.7502-1)
An Amended return is also considered filed on the postmark date (REG 301-6402-3(a)(5))
Returns and payments postmarked after a due date are considered filed or paid on the date received by the IRS.