Once the emails have been sent, there is no problem in deleting them. Actually is a smart thing to do (if you dont need to keep e-mails history, of course), it will free a lot of database space. Remember to shrink your database and reindex your tables after that.
Great, thank you! Maybe I can set to NULL the body field, so at least the log is mantained... But I don't understand: how this information is available on the site? I can't find any place where emails are visible (say, customer page, order page, etc.). Is it? Thank you