ID #1025

Sometimes my incoming mail is a little delayed. What's going on?

Some mails may be delayed because of greylisting. Greylisting is an anti-spam technique that's quite effective. Here's how it works:

- Bob, a person who's never emailed you or anyone else at GeekISP before, send you a mail.
- Bob's mail server connects, but GeekISP's servers say "there's a temporary problem, try again a little later".
- After 15 minutes, if Bob's mailer tries again (which it will), GeekISP's servers allow the message through and add the mailer to the whitelist.

Now, given that mail servers retry at different intervals, that can end up being closer to an hour. It really depends on how the sending server is configured.

There is also some software in place to check for messages sent from "pool mailers" ... places like Yahoo and GMail operate them. The software attempts to identify and whitelist those IP ranges.

Why is greylisting effective - you might ask? Often spammers run fire-and-forget mailers that will try and send as many messages as fast as possible. If they receive a temporary failure - they never retry, they just move on.

Another thing to note is that if you send a mail to someone, that mail server is added to the whitelist automatically. Assuming the receiver replies using the same mail server, there should be no delay in the conversation.

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Last update: 2005-03-21 10:27
Author: Dave Steinberg
Revision: 1.0

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