This was reported as http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=718214. I am the original submitter. Essentially: A session left for ~8 hours suddenly started spewing "Unable to store downloaded mails. Append failed" messages, and stopped storing downloaded mails. Restarting akonadi server helped, with tons of error messages in the restart command output (posted on the Debian bug). Reproducible: Didn't try
Sorry for spamming, forgot to mention here: bug 273445 may be related.
> Unable to store downloaded mails. Append failed This sounds like the MySQL server went away problem. There was a problem with MySQL randomly dropping connection after long period of inactivity. This has been fixed in 1.10.3. Please reopen this bug if you should run to this problem again.