SAP Note 26400 - No messages after deleting & creating mail user

Symptom:

WF Message Control controls conditions and so sometimes calls up a mail dispatch program,
if an addressed mail user is deleted and created again, this user cannot receive mails from Message Control.

Cause and prerequisites

Message Control does not administrate mail users according to their user names but uses an internal 10-digit number which is derived from the internal ID of the mail user. This ID is based on the point in time that the mail user was created.

Solution
Access condition records maintenance and type over the name of the mail user. This will set up the current internal ID of the mail user.