Email Problems - A strange problem not letting me access my email

For some reason, I keep getting the following error message when trying to access my email:

There has been an error transferring your mail. I said:
PASS <shhhh! Don't tell anyone.>
and then the POP server ([email protected]) said:
ERR Flush of temp pop dropbox /var/spool/mail/.webmaster.pop failed (28)

Assuming that I have root access to the server, does anyone know how to fix it?

Thanks,
Yevgeniy