Not a game breaker but annoying since it happens on a regular basis and has been this way for months.
Happens every day on my crafter that gets 5 messages with mats: I get 2 or 3 and the message that my mailbox is full when it isn't. The rest come after going in a dungeo, logging in to another character etc. so its just a temporary glitch.
Any ETA on when you'll get around to fixing this?