Here's what happened to me. Not sure if its similar to anyone else.
I was looking to get the Visage of the daft trickster. Quest is Favor of the Orcakar players. The write ups show 2 quests needed for this. Postmasters challenge and Fifth Nybright Sister.
Having never done anything for either of these quests, I tackled the sister quest first. Went smoothly and received the armored orcakar figurine.
Next was all the running around for the mail deliveries. ally faction with the bards.
I was very organized and did all 18 hand ins in 2 playing sessions. Made sure to get mail and give to correct people. Each hand in I received exp/faction hits and the beloved quest completed horn sound we all know and love. I didn't notice any postmaster mail updates after any hand in though, which may be something. I also didn't talk to the postmaster before doing all the mail, though someone on the write ups has shown it was not needed. after all the successful hand ins, i went to the Postmaster, said challenge or whatever to him and he shows 0 of 18. Dang it #$%&$
So Im like guess i have to do it all again right, nothing counted? I go to the npcs again to try and get mail, and nothing. They say what I mentioned before in my previous post. So now I'm stuck. This is a legit bug. The quest may be working as intended but it is possible to bug it out, which needs to be looked at.
Someone who stumbles on these mail tasks and does them doesn't know its part of something bigger and essentially locks themselves out. If anything, they shouldnt give any mail to anyone until the postmaster part is done first? assuming thats the issue, it may not be.
Edit- adding I have never done Going Postal quest before this. Reading this thread, I see there was a previous quest that may have been mixed up with Postmaster's challenge flags.