Mitigating User-Facing Effects of Mailbomb Attack
If the attack is still going you could forward your email to a Gmail account. Gmail has excellent spam filtering built in. Or you could try to find something that all mailbomb-mails have in common and create a spam filter rule to block them.
If the attack is finished, you may be able to download all your mail from the server, delete the mailbomb-mails (e.g. you can use grep to create a list of them if they have a phrase in common), and reupload them.