Modify stored mail contents?

[Evan Martin]

Is there a safe way to modify the contents of emails stored by Dovecot? 
I'll probably only want to change the message bodies, not the headers, 
if that matters. Looking for ways to do this both for existing emails 
and new emails as they are received (though anything that works for 
existing emails can probably just be run again for new emails.) My mail 
storage is currently mdbox, but I could migrate to another format if 
that helps.