Recently on the Email Marketers Club there have been discussions surrounding Yahoo! and the possibility of Greylisting being used as an anti-spam feature on their systems.

What is Greylisting?

Greylisting is a simple method of defending electronic mail users against e-mail spam. In short, a mail transfer agent which uses greylisting will “temporarily reject” any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again to send it later, at which time the destination will accept it. If the mail is from a spammer, it will probably not be retried.

How do you know if someone if using Greylisting?

Generally greylsiting is visible in your email logs when you see a response code similar to this ” stat=Deferred: 450: Recipient address rejected: Greylisted”.

What can I do about this?

Nothing generally needs to be done to correct this. Most Mail systems will retry a message by default after a set number of minutes or hours. Spam systems and bots tend to ignore these types of notices and are not able to get around these grey listing systems.

Is Yahoo! using Greylisting?

Yahoo!’s mail help pages states that they are not greylisting mail servers sending messages into their systems. However they have increased the level of filtering on their mail servers and have implemented a number of system rules that may cause your mail to be deferred for a period of time, usually a couple of hours.