I had the described problem also. In my case has been solved by freeing up some space at the mailbox location. I had exceeded the disk space quota and, understandably, the sent mail could not be saved.
Lukas Chvatal -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
