I looked through the mods but I don't see anything related. I can ask if someone has something in the works that would allow for moving PM's to e-mail.
One thought I had which is a mod on Merc's is to have the forum e-mail a PM, or a range of PM's to the e-mail address listed in the users profile. My only concern with this is spamming. If Tannin was to send all 50 saved PM's to his inbox, that would generate a load on the e-mail system. I don't know if the provider would complain, but it's a thought.
As a temporary solution I've increased the inbox to 100 saves (it was 50). I didn't know this was a problem, thanks for bringing it up.
We aren't full of members right now, so this shouldn't be a performance hit. The database is big for the number of members we have, but we have plenty of room to grow.