We have an inbound email service that is very restrictive on who is allowed to send through our Sendmail gateways. The restrictions are generally controlled through the access file by marking certain servers, IPs or networks as OK -- everything else is rejected.
This method becomes difficult to control when customers are using hosted solutions like Google Apps or Office 365, where mail could come from any number of servers or networks.
In order to get around that, what I would like to add as a feature are SPF checks on certain sender domains. If their domain passes SPF, then they are allowed to send to us; otherwise the message will be rejected.
It seems there are a few milters out there to handle SPF, but documentation is sparse.
So the question...
What options do I have for SPF filtering in Sendmail which can be limited to certain sender domains?
Well there is spfmilter and milter-spiff just to name two. And milter-spiff's documentation is far from sparse. We're using for many years now without any problems.