20 CFR § 320.49 - Determination of date of filing.
(a) General rule. Except as otherwise provided in paragraph (b) of this section, for purposes of this part, a document or form is filed on the day it is received by an office of the Board or by an employee of the Board who is authorized to receive it at a place other than one of the Board's offices.
(b) Other dates of filing. The Board will also accept as the date of filing the date a document or form is mailed to the Board by the United States mail, if using the date the Board receives it would result in the loss or lessening of rights. The date shown by a U.S. postmark will be used as the date of mailing. If the postmark is unreadable, or there is no postmark, the Board will consider other evidence of when the document or form was mailed to the Board.
(c) Use of electronic mail. By agreement between a base-year employer and the Board, any document required to be filed with the Board or any notice required to be sent to the employer may be transmitted by electronic mail.