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7. Malware Protection 

http://fehcom.de/sqmail/sqmaildoc_07.html

7.5 Spam Prevention 

Once, a handle regarding sending from bot-nets is given, control can be re-established. Two means are used for this purpose:

  *  Greylisting: The sender is initially greylisted and a sending attempt is accepted only after n>seconds. This requires to set up a state-table per connection attempt.
  *  Greetdelay: The sender is 'halted' for n seconds before sending the required 'Helo' greeting from the server. This feature discourages any illegitimate sender since it sucks up resources to send the mail.

Both methods work equally well; but both have their pros and cons. Using rblsmtpd Greetdelay is supported and works incredible well; without the need to maintain a state table for SMTP connections.