I'm thinking that SA might also benefit from a call-back plug-in that looked at the MAIL-FROM and various 822 addresses, opened a connection to the mail server for the domain[s], and verified the sender's address as valid. This would actually be a fair bit of effort given all the stuff that has to be done (MX and fall-back processing, connection management within a time-limit, etc). I'm also aware that some people really dislike these things. The real question it seems is the amount of spam something like this might catch.
I've done some poking in google but can't seem to find trustworthy numbers and experiences. Anybody got any thoughts here? -- Eric A. Hall http://www.ehsco.com/ Internet Core Protocols http://www.oreilly.com/catalog/coreprot/
