My dad has a POP email account, and has been using Evolution to manage it since Karmic or Lucid (with no problems). After upgrading him to Oneiric a couple days ago, when I opened evolution, it asked to convert his mailbox format because the format had changed. (I think it wanted to switch from mbox to MailDir, but I can't remember.) I allowed it to convert—that was my only option—but now whenever he sends mail he gets this popup (screenshot at end of post):
Your message was sent, but an error occured during post-processing.
The reported error was "Failed to append to mbox:///home/shopmaster/.local/share/evolution/mail/local#Sent:
Invalid folder URI 'mbox:///home/shopmaster/.local/share/evolution/mail/local#Sent'
Appending to local 'Sent' folder instead.".
ls listing at /home/shopmaster/.local/share/evolution/mail is:
1260822881.20004.1@shopmaster-laptop local local_mbox pop vfolder
Launchpad bug #850106 also deals with this issue, but I thought Ask Ubuntu might be a good place to get answers as well.