Mail servers' proxy (imap+pop3+smtp and WEBMAIL)

Hello,
I'm trying to configure a full proxy for my mailservers.
With full, I mean for all protocols.

We have, for example, 2 mail servers:
zimbra (imap,pop3,smtp,webmail)
linux+roundcube (imap, pop3, smtp, webmail)

Using MySQL I'm able to configure imap,pop3,smtp proxy: when a user connect to the proxy, my nginx script checks on a database where to redirect the login. If to the Zimbra server or to the linux server.

The problem is with webmail: is there some way to redirect http to zimbra webmail or to roundcube, depending on the username? Do I have to create a login page and then use webmail's api to redirect the login?

What I want is to give a single hostname to users, for example webmail.mydomain.com, and then let nginx redirect users to the right webmail how it does for imap,pop3 and smtp.

Thanks!
Andrea