The very first time it happened X is likely off the hook.

However notification plays a role here, there’s a bunch of things the post office does if someone tries to use them to do this regularly and you ask the post office to do something. The issue therefore is if people complain and then X does absolutely nothing while having a plethora of reasonable options to stop this harassment.

https://faq.usps.com/s/article/What-Options-Do-I-Have-Regard...

You may file PS Form 1500 at a local Post Office to prevent receipt of unwanted obscene materials in the mail or to stop receipt of "obscene" materials in the mail. The Post Office offers two programs to help you protect yourself (and your eligible minor children).