Does anyone know in which Seinfeld episode Newman explains why postal
workers go crazy. "Mail, Jerry, it never stops.."?

Dear chiaberi,

It's the episode "The Old Man", originally aired on 18 February 1993.
The dialogue you refer to is:


Newman: "I'm a United States postal worker."

George: "Aren't those the guys that always go crazy and come back with
a gun and shoot everybody?"

Newman: "Sometimes..."

Jerry: "Why *is* that?"

Newman: "Because the mail never stops. It just keeps coming and coming
and coming, there's never a let-up. It's relentless. Every day it
piles up more and more and more! And you gotta get it out but the more
you get it out the more it keeps coming in. And then the bar code
reader breaks and it's *Publisher's Clearing House* day!!!"


Hope this answers your question!
Regards,
Scriptor


Source:
Seinfeld Scripts: The Old Man
http://www.seinfeldscripts.com/TheOldMan.htm