Patrick Muldoon <doon@inoc.net> said:

If an ISP who doesn't have reverse DNS setup correctly on their mail servers, we point them to the RFC's and generally offer to help them correct it. We have noticed that our spam has reduced drastically, and the complaints are few, but alas this is a double edged sword, where if you even block 1 legitimate e-mail out of the 100K+ that we receive daily, someone is going to complain.

To be honest, I would do this as a last resort. For example, if their ISP's (or their own, if they have the block delegated) DNS server is misconfigured, etc, you might have a issue. Also, some admins have no clue what reverse dns is, seriously :). I would use a service like MAPS first to see if that resolves the issue, and use this as a last resort. If they don't have an issue with blocking unresolvable ips, then they shouldn't have an issue with just using an rbl ;). --conradr