Limiting login domains by virtualhosts?

[tomoki]

I am setting up multiple domains on a single Zimbra setup.
lets call it:
domainA.com
domainB.com

I've setup VirtualHost for both domains
domaina.com
http://mail.domainB.com
so that userA@domainA.com can login as userA at mail.domainA.com
and the same for userB@domainB.com.

everything is working great, except that userB@domainB.com
can login using the domaina.com by entering the full account name
userB@domainB.com. Is there anyway to limit via the virtualHostname, so that
users of one domain cannot login using the virtualhost URL of the other domain?

TIA,
Tomoki