Just an FYI for you kids that like to experiment.
I cleared out EVERY email message on my Pre and emtied EVERY trash can, which in theory would mean I wasn't using any space in my email storage. Then I checked it...
I used mvapp to tell me that I had 50 MB of email and 41 MB of attachments on my /var.
So I used mvapp to move it all to the media drive area. where it showed I had 177MB of email and 136MB on attachments on my /media.
Then I used Internalz to verify there was a lot of crap on/media that I thought I had deleted. I planned on deleting my e-mail accts and starting over. Instead, I started deleting the buckets, one by one with Internalz. As I recall, there were exactly 20 buckets for mail and 20 for attachments. After all of the deletions, this left the email and attachment folders empty. No buckets, no notes, no attachments. Then I used mvapp to move the mail back to var.
The whole Pre seemed to run better. It seemed snappier, email worked better, I downloaded programs like crazy. In fact, I had 90 apps on my phone. After receiving and deleting emails for a few hours, I went back into internalz, and found that the buckets were starting to come back, slowly but surely. Only this time, they had current stuff in them. Not the months and months of mail that I thought was deleted.
My moral to the story is clean up your mail and delete the buckets. Then go back later and clean up your email and delete the buckets again. Depending on your usage, you may want to do it every month, or maybe more often or less often. Anyway, I am convinced it should be a part of periodic maintenance. The buckets will repopulate as required.