my tenants girlfriend which should not be living here due to rental agreement. she is checking my mailbox.?

If you have given the tenant permission to use the mailbox then his guest can check the mailbox at his request. If the lease or rental agreement restricts the occupancy of the apartment to only one full time person then you can advise him that she needs to leave but if the language(fine print) is ambiguous you may be stuck with both of them. Good Luck

If you and your tenant share the mailbox, it is not illegal for your tenant's guest to open the mailbox & take your tenant's mail.

I would suggest that you check the mailbox first to be sure the girlfriend is not receiving mail there. If she is, she has established residency and you have bigger problems than her opening the mailbox.

This doesn't sound like you have much of a case, you would have to prove she is living there. Your tenant is allowed to have company, and you guys share the same mailbox. He can simply say I asked my girlfriend to check if I had any mail. You should have separate mailboxes and maybe you would have a better case. Good luck!!

on it sites are good top links for decisions of your problem http://all-info.org/ans.php?ques=8AAWwQE