Postmark
/ˈpəʊstmɑːrk/
Definition 1 of 2
Pro Tip 1/3Historical Evidence
Postmarks can provide historical evidence for the sending of letters or parcels. 
The old postcard's postmark gave us a clue about the era it was from.
Pro Tip 2/3Authentication Purpose
A postmark can serve as proof that an item was mailed by a certain date. 
To confirm the tax return was sent on time, the IRS checked the postmark date.
Pro Tip 3/3Collectible Aspect
Some postmarks are considered collectible and can be valuable to philatelists. 
He collected letters with rare postmarks as a hobby.