If you add an “A” record for webmail.theirdomain.com to point to the interworx server where their mail is, they should be able to access their webmail at:
I realize the “/webmail” at the end of the url feels redundant, but that’s the way the webmail apps are accessible in the webserver config.
The next question then becomes “well, what will show up if they just got to http://webmail.theirdomain.com?” and the answer will vary depending on the “Default Sites” setting for that IP in NodeWorx > Server > IP Management > Default Sites. By default, it’s configured to show whatever is setup at /var/www/html/index.html - you could modify that to be a generic landing page with a link to /webmail - or even a meta-redirect to /webmail.