[eigen]

from the IRS (https://www.irs.gov/filing/individuals/when-to-file)

Your return is considered filed on time if your envelope is properly addressed, postmarked and deposited in the mail by the due date.

[Tempest1981]

It's strange to me that they list them in that order... making me think they meant "addressed, stamped, and deposited", since postmarking clearly occurs later, after the others.