Unless you have suffered some sort of financial loss filing suit is probably not the best course of action. If someone is stealing your mail, first contact the post office in your area and ask to speak with regional director, or branch director. This ultimately would be a federal criminal matter.
I agree with the other attorney that responded. I believe you can report it to your postal inspector as well.
The actual offense is tampering with the mail and it is a Federal crime. Contact the postal inspector and have your evidence organized to prove the offense