The way to make these changes is by modifying the mail file template to filter out the OU and Domain name information. As long as all your users are in the same OU or Domain, you can just display the CN information. This is a pretty straightforward change BUT not one to be taken lightly. Once you start modifying the mail template (which you should always do by copying the Lotus provided template into your own custom template), you now have an added step for each server version upgrade. It is best to keep the templates current as Lotus still add a lot of fixes to the mail component of the system. Make sure the change is VERY documented so it is easy to add to each future release.
What version of Notes/Domino are you using?