What is Greylisting and how does it work?
Greylisting is a new tool in the fight against spam. What it does is temporarily
block incoming mail from a sender. It returns the mail to the sender's mail server
with a message saying try again later. The sending server must then retry sending
the mail after the Block Period of 10 minutes but before the Pass Period of 6 hours (see below for definitions
of these values). The blocking server will keep a record of approved emails and will not block them a second time. If a sender does not send any mail to the blocking server for a period of 6 months, the record of the approved email is removed and any mail from this address will again be greylisted the next time mail is received.
Greylisting is effective because spammers will not usually bother to attempt a second
delivery, but legitimate e-mail servers will.
Why use Greylisting?
Greylisting is a very effective method of spam blocking that comes at a minimal
price in terms of performance. Most of the actual processing that needs to be done
for Greylisting takes place on the sender's server. It has been shown to block upwards
of 95% of incoming spam simply because so many spammers don't use a standard mail
server which will do automatic retries.
Disadvantages of Greylisting
The biggest disadvantage of greylisting is the delay of legitimate e-mail from servers
not yet verified. This is especially apparent when a server attempts to verify a new
user's identity by sending them a confirmation e-mail.
Some e-mail servers will not attempt to re-deliver e-mail or the re-delivery window is
too short.
Article ID: 9, Created On: 10/2/2008, Modified: 10/2/2008