What is Greylisting?

Greylisting is a relatively new method of blocking SPAM at the mailserver. Rather than resort to extensive statistical analysis or banned keyword comparisons, it uses methods based on the time-driven component of SPAM that is predictive of most spam. SPAM email does not generally behave like normal email systems, and as a result Greylisting takes advantage of this to be able to block most SPAM. Greylisting describes a general method of blocking spam based on the behavior of the sending server, rather than the content of the email.

Why did you bounce my email?

Greylisting works with standard SMTP email servers to cause the first email meeting certain criteria to be temporarily delayed based on the SMTP spec­i­fi­ca­tion. This imple­men­ta­tion is very restrictive and only does this the first time email is received from a particular domain from. This should be transparent to the sender.

If your email is bounced after attempting to send an email, it is because the sending server is not configured to properly handle 45x error codes and delivery retries. You should refer your email technical staff to the links below, which should solve your problem. You can also contact the Postmaster for more information. We are not trying to block legitimate traffic, and we would love to work with you to resolve this problem.

Can I use Greylisting on my own mail server?

Yes, there are imple­men­ta­tions of Greylisting available for many types of mail server software. See the links below to further resources on Greylisting: