Comment 1 for bug 376808

I got the same thing, and I'm pretty sure you're right -- it occurs if it tries to check your folders before the wireless is settled. If you want an easy fix, it is possible to alter the startup command to make MN delay for a set amount of time before starting. This has worked perfectly for me.

In my startup command for MN I have:

sh -c "sleep 60; mail-notification --sm-disable" &

The sleep is the wait command. The number is in seconds, so change to anything you want. I'm not sure what the rest does, except that it works! At least in fedora, and presumably in ubuntu, since I got it from this forum:

http://ubuntuforums.org/showthread.php?t=1175502&highlight=delay+startup

Good luck!