27 CFR § 53.154 - Manner of filing returns.
(a) Each return on TTB Form 5300.26 shall be filed with TTB, in accordance with the instructions on the form.
(b) When the taxpayer sends the return on TTB Form 5300.26 by U.S. Mail, the official postmark of the U.S. Postal Service stamped on the cover in which the return was mailed shall be considered the date of delivery of the return. When the postmark on the cover is illegible, the burden of proving when the postmark was made will be on the taxpayer. When the taxpayer sends the return with or without remittance by registered mail or by certified mail, the date of registry or the date of the postmark on the sender's receipt of certified mail, as the case may be, shall be treated as the date of delivery of the return and, if accompanied, of the remittance.