I too seem to be having this issue. Running Perfect Ubuntu setup and just upgraded from .24 to .27 hoping to fix the issue... No luck.
I started off being able to send but not receive email, then through a number of trial and errors figured out DNS was an issue (now resolved). Now I send emails to the server and they are no longer getting bounced back as an undeliverable mailbox.... but I also never actually get the emails..
Any help would be greatly appreciated!