>I forgot to add, can't a dns check fail because rejected just like spam? Would AOL let you (and spammers) check via dns tons of AOL emails? Maybe technically unable to stop it as you disconnect as soon as connected, but maybe not. Anyone know?
I can give you an idea of what we do to try and keep the email-baddies at bay.
DNS itself is harder to check and control than a mail server but the email address style that uses the domain name ([EMAIL PROTECTED]) actually is a wildcard DNS entry (*.domainname.com in the example above) pointing to the mail server and the mail server then works out what the account actually is. So doing a DNS check will not help much in verifying an account other than flagging that the domain name actually exists.
On our mail servers we monitor the IP address of every incoming mail server or email client that talks to ours and check the frequency of connection. If we get a burst of many connections in a very short space of time we ban that IP address for an hour then turn it back on again. It doesn't affect legitimate connections and if someone is sending many dozen emails at once (very unusual) then they will eventually get sent in bursts of a few at a time depending on the thresholds used but we have found that illegitimate connections, such as a script kiddie scanning for valid POP accounts, will pick up the banned status and move on to the next mail server in their list and leave us alone.
As a Hosting provider we have a very large number of domain names and related email accounts in our system which gives a potential attacked or SPAMmer a rich harvest so we need to keep things as tight as possible. Looking in the mail server's admin interface right now I see just over a hundred banned IP addresses and if we look up most of those they turn out to be at the other end of a DSL connection so are probably personal computers that have been attacked by a trojan horse and are sending SPAM/viruses without the owner knowledge.
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Yours,
Kym
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