"Must be postmarked by midnight on Nov. 26th": does anyone really check?

If you’ve ever worked for a company that sets deadlines like this—for changes of insurance benefits, enrollment in programs, contest entries, etc.—tell me: does someone actually look at the envelopes and read the postmarks?

Or is there just a received-by date set, allowing enough leeway so that the nominal deadline will have been met?

And is “received by” defined as “in the door” or as “posted by a clerk in some department a long, long way from the mail room”?

In the latter case, are the envelopes of late arrivals checked, to cover the case of postal delays?

Who gets the benefit of the doubt?