hmmm. Try accessing it by using your panel login URL (panel.domain.com)/etc/apps/webmail/
If that works, then it could be a .htaccess lockdown. Me.B is more familiar with what they did to lock things down and could advise better
If that works, then it could be a .htaccess lockdown. Me.B is more familiar with what they did to lock things down and could advise better