Occupant Rights

Is the person living lawfully in the apartment and are you talking about regular USPS mail?

unenforceable- they going to examine the mailman's bag?

First of all, USPS mail is tightly controlled by USPS. It's generally a Federal offense to interfere with someone else's USPS mail. Practically speaking, if you're in one of those rare buildings that still have the elevator man sort and deliver the mail to your apartment door, or you rely on the doormen to hold packages for you, it might not quite be the same situation.

Moreover, though, http://www.housingnyc.com/html/guide/basics.html#Roommates

If you're that 19-year-old on another current thread, note that "roommate" implies that the leaseholder is actively living in the apartment.

Yes regular postal mail...The person is not on the lease but living in the apartment with the lease holder