What is greylisting?

Greylisting is a method of reducing spam by exploiting a difference between spam email and real email - the fact that spam mail is not resent by the outgoing server if it is rejected the first time.   This reduces the spam you will receive by up to 70%.

If you have greylisting switched on, some emails that have been sent to you may take up to two hours to appear in your inbox. This occurs because emails that might be spam are rejected by the incoming mail server the first time they are received. Standard Mailservers will always automatically resend this email. When the incoming mailserver receives the email for a second time, they are accepted. (see diagram below) 

This works to remove spam because spam mail is not usually resent by the server.  It is possible (though unlikely) that legitimate mail may not get to you if it is coming from an incorrectly set up server, which doesn't resend mail. 

Please contact Datasearch if you have any questions about greylisting or wish to try it out.