Maybe this will be useful to someone. Yesterday i noticed that 2 of my PMs stayed in the “Outbox” instead of moving to the “Sentbox”. After some poking around, i found out that my “Sentbox” was 100% full, so there wasn’t space left for the new messages to be copied there, so apparently they just weren’t being sent.
I deleted everything from my “Sentbox” and the messages were sent.
So, check your “Sentbox” and clean it up from time to time…
What the little elves do with the messages and when is still a mystery to me, but my theory is that my messages were staying in the outbox forever because the sentbox was full, i.e., the recipients weren’t seeing them.