moderated Re: Deleting attachments when out of space #update
How about deduplication first? Eliminate multiple instances of the same file--perhaps even annotating the messages to indicate where the file has been retained--reducing space with no loss of information.
To carry this thought a little further (and this may be out of scope), there are schemes where files differing only slightly have the common parts deduplicated. Riverbed was one of the first firms to provide hardware appliances that do this, but now it's a complete industry. See Deduplication Appliance Buying Guide
This function can also be performed in software. See Data Deduplication