https://en.wikipedia.org/wiki/Rowan_v._United_States_Post_Of...

> The addressee of postal mail has unreviewable discretion to decide whether to receive further material from a particular sender, and a vendor does not have a constitutional right to send unwanted material to an unreceptive addressee.

It's not necessarily that the receiver has the sole right to determine if the material is pornographic or whatever, its that the receiver of mail has the right to decide to no longer receive material and that the sender doesn't have a right to force its delivery through the mail.

The form to prevent someone from sending you mail you don't want is a PS Form 1500. This form starts off saying:

> If you are receiving unwanted sexually oriented advertisements coming through the mail to your home or business

But, you can still just file it against say a roofer sending you unwanted advertising or whatever. The USPS isn't allowed to challenge your personal determination that you're receiving unwated sexually oriented advertisements. Maybe you personally find roofers sexy and are trying to avoid being around roofers and having their services offered at your home. USPS isn't allowed to judge.