Look at the archiving tools for the mail database. under the 'archive criteria' there is a section on 'what should happen to the documents?'. just removing them is one of the options. You could schedule the archiving to happen on a regular basis, or just run it manually.

There is an option to purge all documents not modified in the past 30 days. You can purge older documents (including calendar items) by clicking on Replication Settings > Space Savers and then check the box to the left of "Remove documents not modified in the last (days)". The number you will see is 90 but this can be changed to 30 if that is the cutoff interval you're following. Click the OK button afterwards and the older documents will be purged. You do not need to repeat this action once set on a given mail database.