I have already talked to my landlord about it and he seems unconcerned. Is there anything else I can do?
This morning, I called the post office who referred me to my area's postal inspector. After filing a complaint, they told me that it was not a matter that they took lightly and that they would have inspectors look into it. The person also mentioned that they will try something akin to what my dad did with sending a fake letter to entrap him.
When I got home, I wanted to make sure my landlord got my rent check because it hasn't went through yet. He told me that he has not forgotten about the mail issue and that a lock will be installed within two weeks. I told him that I contacted the postal service and he agreed that it was the right thing to do.
Hopefully, the issue will be solved soon.
Thanks everyone for your input. Oh, I also put a note inside my mailbox saying, "DO NOT TOUCH MY MAIL - thank you". Haha.