That is actually the perfect analogy, only not as you intended. The post office lost your mail so you get them to fix their problem, you don't contact the recipient and say change your address. Thanks for clearing that up, you weren't on the debate team were you?
Except in the case of the op, the system is working fine. imessage is designed to use the email addressed in addition to numbers. He's upset because people he's communicating with don't / won't use their devices correctly / as intended. But he seems insistent on blaming himself as the root of the problem, hence the metaphor.
If those people were making proper use of contact entries this "non issue" wouldn't exist.
Hopefully you weren't on the debate team either. Context matters.