multi-domains: imap&smtp need xxx@domain?

[ypong]

I've got ZCS 7.1.2 OSE with proxy and multi-domain running, and I've created 3 different domains with the respective IP addresses e.g.
domainA 10.10.10.1
domainB 10.10.10.11
domainC 10.10.10.21

I've also provisioned the zimbraVirtualIPAddress for the respective domains using zmprov md, and double checked that zmprov gd domainX is appropriately set for zimbraVirtualHostname and zimbraVirtualIPAddress.

Webmail is fine through e.g. http://domainX.com to the mailstores (via the proxied ports of course).

But for imap and smtp access, if I setup thunderbird with the user account, I cannot authenticate properly (it just says authentication failure), unless I use the form "user@domainX.com".

i.e. I cannot just use "user", I have to use "user@domainX.com".

Any idea why this is happening? I would expect that with different IP addresses mapped to the different virtual hosts, Zimbra/Nginx would be able to identify (as it does, through the webmail interface) which domain I'm coming through (my DNS and reverse DNS entries check out correct), without having to specify user@domain.