Nathan,
If your subscribers have fixed ip addresses, you can whitelist their addresses. I have done that successfully for several groups that I host here locally.
Unfortunately fixed IP is not as common for residential services, so this may or may not apply to your subscribers.
For folks on variable IP services, I am looking at installing two separate SMTP paths, as I have a similar problem facing one group that I support overseas. One path would be "public", anonymous (non-authenticated) access, and filtered by SpamFilter. The other path would be "private", authenticatated SMTP, and not filtered. I'm planning to use two host names, each pointing to a different IP.
Alternatively, you may be able to take advantage of typical DSL IP assignment strategies in order to limit your exposure to whitelisting IP addresses. When solving a VPN-related problem, I observed that cable and dsl routers assign IP addresses across class B or C address spaces. Mostly I saw addresses vary within a class C. I cannot speak to the general validity of this worldwide, but I've seen it both in US and Aus. My solution was to allow connections from the class C range using wildcards. Our equivalent with SpamFilter would be to whitelist the class C in which each subscriber operates. If you have any volume of subscribers, the 2 path SMTP solution is simpler to administer, as there's no way to keep track of "who uses what IP range" once you've added the range to the IP whitelist. If you have only a small number of subscribers impacted by this situation, however, this may work as a quick fix. You still have the problem that your SMTP server (probably) is an open relay to anyone on any of the whitelisted class C address spaces. You'll have to assess that risk yourself.
Hope this helps.
Robert Shelton
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