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faq:email07 [2011/04/30 17:08] (current)
clemens created
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 +<​code>​
 +[07] WHAT IS GREYLISTING?​ HOW DOES HELP WITH SPAM OR UCE?
  
 +     SPAM that you receive can be forwarded to '​spam-bucket@sdf.lonestar.org'​.
 +     This file is accessible to all users and the purpose of it is to help 
 +     ​identify spammer networks and spam content.
 +
 + WHAT IS GREYLISTING?​
 +
 +     ​Greylisting is a passive approach to dealing with spam.  It allows the
 +     SDF SMTP server to keep track of the SMTP servers that communicate with it.
 +     When SDF receives a connection from an unknown SMTP server it issues a 451,
 +     which basically means "​I'​m really busy, please retry later"​. ​ This sort of
 +     ​response occurs normally for a multiple of reasons everyday such as:
 +     The user is over quota, the file system is full, the load average is too
 +     high and so on.  A properly configured MTA will follow the SMTP protocol
 +     and respect a 451 by using its default retry interval which can be 
 +     ​anywhere between 5 minutes to 60 minutes typically. ​ SDF's greylisting is 
 +     only in effect for 3 minutes from the sending server'​s first attempt. This
 +     is well within a reasonable retry period of a properly configured SMTP 
 +     ​server. ​ When the previously greylisted server connects back within 8 
 +     hours of its first attempt, SDF accepts its connection and allows the 
 +     email to be delivered. ​ The host is then whitelisted for 48 hours. ​ This 
 +     also takes in account for other SMTP servers on the same network since 
 +     ​greylisting on SDF only looks at CIDR 24 (255.255.255.0) and therefore ​
 +     the host number is ignored. ​ This allows greylisting to work with large 
 +     mail harvesting farms such as Gmail.
 +
 +     ​Senders with SPF compliant headers are automatically passed without
 +     being deferred.
 +
 +     You will be able to see the header line X-Greylisting in your email
 +     which will provide you details on the host and perhaps can help you
 +     with any additional filtering you might need.
 +
 +     There is a simple utility called '​greylist'​ you can use to see what tuples
 +     apply to you.  Its important to note that if you do see a tuple in the 
 +     ​greylist that you known is legtimate it will always show up in the 
 +     ​autowhitelist,​ for 36 hours, when the sending host retries. ​ Because it
 +     is possible that a spam host could resend before they change their IP 
 +     ​address,​ you could receive that spam on a retry. ​ However, it is unlikely
 +     that they will retry and therefore you will always receive legitimate ​
 +     email with a very low percentage of that possibly being spam.
 +
 +     By default greylisting is enabled for all SDF members. ​ If you would 
 +     like to disable it, which is not recommended,​ you may do so by typing
 +     '​greylist -t'​. ​ You can re-enable it with the same command.
 +
 +     ​MetaARPA members can also use the '​greylist -tw' command to create their 
 +     own rules to apply to mail delivery specific to their email addresses and 
 +     their domains. ​ You must have greylisting enabled otherwise there will be 
 +     no need for a whitelist. ​ This custom whitelist is a flat text file called ​
 +     .wl in the user's home directory with a single email address on each row 
 +     of the file.  The file can only contain email addresses and 
 +     ​meta-characters will be stripped and ignored.
 +</​code>​
 +
 +[[email|back]]