I am not aware that . It is. There have been publicized cases of postal workers perpetrating violence on others. I really don't know the answer. But a high percentage of postal workers are retired military or military reserves. I wonder if violence has any thing to do with past traumatic combat experiences. I have nothing to back that up with - but your question intrigued me.
Suicide. Being a postal worker is very much akin to being in the military. For many it becomes a thankless job with dwindling importance with the invention of the internet, and with the USPS on the verge of bankruptcy and drowning in red-tape, it has become an incredibly unhealthy place to work over the years, demeaning, and draining for those who work there.