where can I find my professor's mailbox?

The mailboxes of the professors are located at the front of the corridor of the professor's office.

Usually papers are collected in separate mailboxes at the elevator at the student administration FSW, in building De Meerminne, 1st floor (low building)

The FSW student administration also has a mailbox next to the door of office M.135

If the building is closed, you can deposit mail (in a closed addressed envelope) via the large mailbox on the outside of the building to the left of the sliding door of the bicycle cellar.