Why does exchange keep reverting back to previous settings?

Hello

Using MS server 2003 sp2 with exchange v6.5 i am trying to edit the SMTP details for one of my users. So i find them in the active directory list and open the email addresses tab. I have listed two sets of SMTP details which is fine, dave.smith@email.com and davesmith@email.com and of course X400 for exchange.  

My Problem is that the email is currently set as davesmith@email.com so i set dave.smith@email.com as primary and it turns bold, i then apply and ok out of the window yet when i open the window again and check the settings it reverts back to the previous configuration....

Has anyone come across this before?

a little help about where i am going wrong would be greatly appreciated.

Many Thanks

James