the obscure sexual act referring to gloryholing through a mailbox slot in someone's door. first refereed to in cyanide and happiness comic #2373
steve: hows the mailman going john?
john: awesome, dropped off my package in my neighbor's door yesterday, gave them some junk mail.
steve:aw, tha's nastyyy.
Gloryholing through a mailbox slot.
Guy 1: So you get yo dick sucked last night?
Guy 2: Yeah, but i didn't feel like looking at her face so I junk mailed her.