Which messages appear in the mail log when you send a message from a account on your server to another account on your server?
Furthermore I want to report a bug:
I added a new mailuser in one of my webs, but forgot to set a pwd. So I deleted that email user again. Now it seems impossible to add that email user new again, since ispconfig keeps complaining that the email still exists. I have no clue where to look to remove that erreneous email user, so I can add it again.
Thats no bug. You just missed to empty the recycle bin