Greylisting...

[Rob Poe]

An additional sendmail ruleset stops SMTP transactions with MTAs that 
HELO/EHLO as our MX, with this message: 
          "ruleset=check_rcpt, arg1=<recipient_address>,
relay=[relay_IPADDR], 
           reject=554 5.7.1 Invalid helo rejected; send mail to
abuse at odot.org 
           if rejected in error - are you really 192.149.244.25"
which stops even more _and_ gives me nice patterns to watch in my
maillog 
database. But all that's off-topic here, so ask in private mail if you
want more info. 

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I'd like to hear more about that one :)