How to prevent the duplivate ID

Hi ,

We are having lotus domino POP3 mail server and my clients are accessing mail through netscape messenger where user can easily change the email identity through preference option , the problem is one of my user has changed his email identity as my vp’s email id and he has sent a funny mail to some one of his friend , but unfortunately that mail got bounced back and reached my vp’s mail box , now we have instructed by our VP to find out a solution for this problem .

Is there any options to prevent this type of problems.Please helpe in this regard.

Regards
Sivakumar