[ietf-smtp] test for port 25 of sending MTA - for spam detection

Hi

Just a thought I had for spam detection: what about testing if you could
connect to port 25 on the sending MTA? Zombies behind a NAT would not be 
able to be reached. I think there are some web-based mailers that could be hurt,
but maybe one could make a whitelist  for those. Is this a known
scheme, and what are the pros and cons?

best regards
keld
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