Zimbra as antispam relay only for one domain

[Labsy]

Hi,

is it possible to configure ZCS to serve just as INCOMING antispam proxy for a domain, without knowing the domain users?

I have dozens of regular domains, with mailboxes on Zimbra, but also have 2 or 3 customers, which have their own mail server, but want to get rid of spam. So my idea was to add these domains to my Zimbra box, and just scan mail for spam and viruses, then let the healthy mail relayed to customers mail server.

But when I tried AUTH mechanism got on my way - I cannot manually add user accounts, since they change too frequently, nor can I use any other auth mechanism, because they won't open their Exchange server to the public.
Any way without auth, just antispam and antivirus scan?

[Labsy]

Bumping once. Maybe someone with knowledge will see my post...

[apatosaur.9]

Bumped by me too, because this is a use-case for me too.