My proposed configuration configures dspam to operate in a blackbox mode, much like the relay.txt instructions. It doesn't matter what backend you use, or what SMTP server you use on the front end.
First off, an overview of my configuration, with justifications.
Inbound SMTP Servers
All incoming e-mail to my domains (I manage a corporate network of 48 domains, many of which are aliases) goes through dedicated servers running:
* qpsmtpd - a replacement for qmail's smtpd, written in Perl using a flexible and extendable plugin architecture; I'm actually running a forking server, but alternative servers include an Apache-managed, a pre-fork server is in testing. You don't have to run qmail in order to use qpsmtpd, as there are queue plugins for most other major SMTP servers.
* clamav - Open source antivirus scanner, running as a plugin to qpsmtpd (using the clamd interface).
* sophos - Commercial antivirus scanner, running as a plugin to qpsmtpd (using the sophie interface); this is our desktop vendor, so it was cheap enough to add a second layer of protection.
* Blacklists - a very few narrowly targetted blacklists, again using a plugin to qpsmtpd.
However, the important thing to note about my configuration is that all e-mail is scanned and accepted prior to submission to dspam. I block what I can (about 60% of my incoming mail) and only pass the rest to dspam. It would be possible to block even more, but in a corporate environment, the acceptance of legitimate mail being blocked is very low.
Backend Mail Server
On the backend, I use:
* vpopmail - a flexible virtual user management program, originally written to support virtual users/domains in qmail, it can also be used with Postfix.
* dovecot - a fast, well designed IMAP/POP server. As of Sept 2006, it is rapidly approaching the official 1.0 release, but it has been stable and used in production for more than two years. The 1.0 release is more a matter of which features have been added, including a Local Delivery Agent (LDA) with support for SIEVE processing.
Again, the important thing to realize is that dspam is configured as a SMTP relay inbetween the front end server(s) and the backend server(s). By decoupling both the inbound SMTP transaction and the eventual mail delivery from dspam,