

What We Found Scanning Millions of Email Systems - bittitan
http://blog.smtplogic.com/2012/10/04/what-we-found-scanning-millions-of-email-systems/

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rogerbinns
My mail server deliberately won't complete in under a second. This is because
many spam systems don't actually speak SMTP - they just spew the protocol at
you and ignore what is being sent back. A little delay helps catch them. (In
Postfix use sleep and reject_unauth_pipelining.)

