Yesterday we experienced a problem with the webmail client at https://webmail.ru.ac.za/. This will have resulted in some people's email being sent with an incorrect address of the form username@imap.ru.ac.za. Note that this address is invalid, and e-mail addressed to it will fail.
The problem has since been resolved. However, it seems that some people's webmail sessions are retaining the incorrect information.
The first thing to do is to completely log out of webmail, restart your browser, and then log in again. This will more than likely resolve the problem.
If it does not, it means that the incorrect information has been saved in your profile. To correct this, log into webmail and go to the Options tab. Select the Personal Information link under General Options. Then choose your default identity (the one listed under Your default identity) from the Select the identity you want to change box. Delete the identity (click on Delete selected identity), then log out and log back in. Note that if you had made any changes to the identity (for instance your preferred name), then you'll need to make these same changes again.