This seems to be a glitch in the email program in Diablo. I wouldn't hold my breath waiting for a fix.

There is a thread on this here:

http://talk.maemo.org/showthread.php?t=25734&page=3

Apparently, email is stored in a cache file, and doesn't really get deleted.

You can go into "x teminal" and manually delete the cached Emails:

It should really be as simple as typing (in x terminal)

Code:
cd ~/.modest/cache/mail/pop/your_pop_mail_account/cache
and

Code:
rm * -rf
No programming required, I promise!

///EDIT

OR if ur stuck by the "your_pop_mail_account" part, to find out what u should type there, just do
Code:
cd ~/.modest/cache/mail/pop/
and then type
Code:
ls
you should see the folder name that you need to put there. Then follow the first step (step 4) again with that folder name in there.