This works with the very simple SI CAPTCHA for WordPress plugin.

I hobe this solve the problem.

Update:
It works.

I’m using a Postfix Mailserver on Debian Etch with amavis and spamassasin.

Fist we want to catch only realy hard spam. Normaly no real mail get an Spamscore over 7 points. Change the required hits in the spamassasin configuration.

vim /etc/spamassassin/local.cf

required_hits 7.0

After that configurate the redirection in the postfix configuration:

vim /etc/postfix/main.cf

# Spam Forward
header_checks = regexp:/etc/postfix/header_checks

vim /etc/postfix/header_checks

/^X-Spam-Flag: YES/ REDIRECT spam@yourdomain.com

/etc/init.d/postfix reload

Now all Mails with the Spam-Flag=Yes will be delivered to the spam@yourdomain.com account on your mailserver. If you miss any important mail, you can check this account.

Now create a script which delete this mails after 7 days.

vim /usr/local/bin/deleteOldSpam.sh

#!/bin/bash
find /path/to/your/mailhome/cur -type f -mtime +7 -exec rm {} \;

After that create an cronjob for this script:

crontab -e

0  1 * * * /usr/local/bin/deleteOldSpam.sh # Delete old Spammails

Now every day at 01:00 all spammails older than 7 days will be deleted.

Don’t forget to modify /path/to/your/mailhome/cur to the path where you store the mails of your spam account user.