You should do as the Colonel suggests to start with. If your landlord fails to comply and lay down the law, you have to go to the courts next and set up an escrow with them and pay your rent there. The courts will then send a letter to your landlord telling them your rent is being held in escrow until he lives up to the rules of the lease. If you have a lawyer they should set it up in short order for you. I know they can be expensive, does your job offer prepaid legal? If it does, you might want to get into that, they are made for just this sort of thing.
As for your mail, if your local postmaster does nothing, I recommend you go over his head to his boss. If he does nothing, go over his head, and so on, and so on, until you get results. Might help if you anonymously inform the news that there is mail theft and the postmaster is doing nothing about it. If there's one thing the media loves, it's bad publicity about something. And if there's one thing the USPS hates, it's bad publicity about themselves.