Constantly on the lookout
It’s no secret that dogs and mailmen are sworn enemies. It must have something to do with the uniform, or maybe there’s been tension between the two since dogs were first domesticated thousands of years ago. Whatever the case, mailmen and dogs don’t mix.
For the OP, the tension between the two got so bad that he would have to actively avoid the mailman and his truck while taking the dog out on walks. Even when the OP would travel alone, if he saw a mailman in the distance, he’d have to turn around and walk away. Whatever bad relationship there is between his dog and the neighborhood deliverer of mail, now the OP was a part of it.