The other possibility is to do use a regular expression algorithm to try to sniff out hosts that look dynamic, though I'm sure there is some legitimate hosts will get nailed.
Are you talking about reverse DNS or HELO? Even the so-called experts just can't get it right with reverse DNS -- many legitimate mailserver IPs have a reverse DNS entry that looks a lot like a dynamic IP (often the exact same format as a dynamic IP from the same Internet provider). HELO, though, is a different story -- occasionally a legitimate mailserver will HELO as something that looks dynamic, but that is quite rare (and easily fixable by the mailserver admin, unlike the reverse DNS that is sometimes fixable but sometimes is not).
-Scott
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