Setup for callout/recipient verifying w/o AD

Hi there,

I am interested in sharing experiences on how to set up recipient verification with callout when Active Directory is not available or unuseable.

We use Lotus Notes and a antispam solution (GFI) in front of the Notes cluster. In the ASG setup the mail server used by callout is the receiving mail server, too - IMHO a not fully baked solution.

I cannot use a Notes server for callout even if this might be possible - mails will flow directly to the Notes cluster bypassing the GFI server. The GFI server does not know anything about our mail addresses, so I cannot use it, too.

Callout is IMHO very important to deny mails to non-existant users at SMTP time. So I created a solution which I somehow "dislike" but it works - another mail server (hmailserver - Freeware) in the middle between ASG and the GFI server which is able to work as the callout server. I have to keep our mail addresses on this one up to date manually, not so funny.
Callout works pretty well, but sometimes mails get stuck when being routed from the hmailserver to the GFI server.

So I am interested in getting informations about other solutions. What do you use?