How long are items/money allowed to sit in the mailbox?

[Jemaniya]

If there is a time limit - what happens to them if the time expires? If the answer is they return to sender, what if sender has been deleted?

[Lorien_the_First_One]

If you look in the mailbox it tells you how long they have left.

Open items - destroyed after 5 days

Unopened items - After 3 weeks:

AH items not claimed are destroyed.

Items sent by other characters are returned to them, and if the sender is gone, they are destroyed.

Items that were returned to you because the sender didn't open them are destroyed.