Linux

I believe you can just setup a server side rule. I know my office does that and you get put in 'e-mail jail' if your box gets too large and no other mail can go in or out. Also, they clean the box of messages older than 30 days so you have to make sure you clean and archive your mail if you don't want to lose it.

If you have the root password, you can do something like the following:

find /var/spool/mail -mtime +30 | xargs rm

Naturally, check first to be sure that's where your sendmail is storing the files (they've got names starting with qf and df, usually).

One sure way is to let your contacts know that you'll be away for 3 months. They won't be sending you mails during that period.

One suggestion is to use Outlook Express together with "MAIL WASHER" program. You can see your mails in your mailbox before downloading. You can choose mails you wish to download or delete the ones that you don't wish to.

Try expiremail by Marc Tremblay. It can be found at http://freshmeat.net/redir/expiremail/36125/url_tgz/expiremail-0.2.tar.gz. You can have it delete mail from any mailbox that is older than a specified nymber of days.