Virtual host redirecting, unable to login with username

[blazeking]

This one is a little difficult to describe, so bear with me.

I have a site (domain.com) that accesses their mail through a virtual host (mail.domain.com) added to the server (server3.realname.org). After login, it redirects to that address. Sometimes, when they visit the virtual host (mail.domain.com), it redirects to server3.realname.org/zimbra/?zlastserver=1 before login, making it impossible to use the username to login. Instead, they must use username@domain.com. Why would it redirect for some users before they log in?