Secondary Domain email account - Webmail login

What is the proper way of logging into webmail using a email address that is attached to a Secondary Domain name?

If I have two domains: [Master domain] [Secondary Domain]

I set up a few email address’s in the secondary domain,, and when the person with the email address goes to and when trying to login is ejected back to the login screen, (yes login info is correct).

Now when they go to and try to login with their email address assigned in the secondary account, this person can login without issue.

So, is there a problem with the login for
Is everything working the way it should and all secondary domains must use the master domain, in this example, to login to webmail?