I have several domains set up for mail accounts. The default domain is mysite.com. Other domains would be theirsite.com and yoursite.com. The server is ALWAYS accessed through mysite.com with the mx records for the other domains pointing to the mysite.com server. Mail delivery works fine to all of the domains. To make things a bit more friendly for logging in and to give us the ability to customize themes we also use virtual domains like theirsite.mysite.com and yoursite.mysite.com.

For some users they seem to work just fine, for others the theme is broken. No images, no formatting. Another users indicates that when he trys to log in under theirsite.mysite.com he gets and error "The theme xxxx is not available please contact your administrator"

All users can access using mysite.com without any issue.

This is a clean install on an new server but the same domains had been used on another server for testing. Could this be a local machine cache issue?