There are post-offices in all the cities and larger towns.
The main post-office in Chania is located in Peridou Street (around the corner of Kydonias Street) and is open from 7.00 am to 8.00 pm Monday to Friday and Saturday morning.
The main post-office in Rethymnon is at Moatsou St. and open from 8.00 am to 2.00 pm Monday to Saturday.
The mail to and from European countries is quite fast, about 2 to 4 days but some letters can also - inexplicably - take weeks.
If you need to send something in a hurry, I have noticed that by sending it registered (cheap at around 3 Euro extra) it seems to get treated as an express letter.
You can also buy stamps at kiosks (and many shops selling postcards) but will need to pay an extra 10%.
A letter to European countries (up to 20g) costs Euro 2.00 (2023).
Some mailboxes seem to get emptied only occasionally (and some very rarely) so if you want to play it safe put them in the post-office letterboxes.