If you want webmail to be configured for the correct domain for the correct end user the first time they use it, then you will need to do that manually yourself before issuing the login details to the user.eglogin to webmail as the end user eg john1 (for domain1) and setup the profile for that user to show the return email address of john@domain1login to webmail as the end user eg john2 (for domain2) and setup the profile for that user to show the return email address of john@domain2Do the same for all other webmail accounts that will be issued configuring the profile and return address as applicable.If you don't configure webmail profiles manually then they will have the default return address of loginusername@domain1 (or the main domain name of the server if different).