Postmark

From Guidevines

A postmark is the Post Office's stamp or mark on a submission via regular mail, indicating the date of mailing.

Usually deadlines for contests or submission periods will indicate whether the dates are "received by" (or firm) or "postmarked by." A postmark deadline will allow one to submit up to the actual date, whereas a firm deadline will mean that punctional delivery is dependent on the speed of the Post Office.

For E-Subs, a postmark deadline is, for all practical purposes, no later than a "received by" deadline.