Problem

Getting a lot of "Remote host: X.X.X.X. RCPT response: '451 4.7.1 Greylisting in action, please come back later".

What does this indicate?

Resolution

The Greylisting is based on the principle that mass email engines don't behave like a regular mail relay (e.g. MailGate). Regular mail relays will, if a temporary error occurs at the receiving end, buffer the email and try again later. Mass email engines, such as the ones powering common affiliate-systems or help-desk systems, normally don't have this functionality built in.

Greylisting generates a temporary error on purpose and waits for the sending relay server to come back later and try to deliver the mail again. In this context:

• A regular mail relay, waits a while, tries again and then succeeds in delivering the mail

• A mass email engine tries one time and then the email is lost forever.

The greylisting mechanism keeps a database of approved senders, each consisting of sender address, recipient address and the IP-adress of the sending email server. When a new combination is found for the first time, a message looking something like this is sent back to the sending email server:

451 4.7.1 Greylisting in action, please come back in 00:30:00

Here we can see the appropriate SMTP codes for a temporary error and a descriptive text of what has happened. If the sending email server tries again before the 30 minutes has passed, it will receive a temporary error again:

451 4.7.1 Greylisting in action, please come back in 00:10:00

After the 30 minutes has passed and the sending server tries again, the email will pass and the sender triplet will be registered as 'whitelisted' for a configurable duration (25 days or something like that). That means that subsequent emails from the same sender to the same recipient will be delivered without delay.

For additional details:

http://www.softwareprojects.com/resources/traffic-attract-customers/t-greylisting-yet-another-reason-why-emails-to-your-1210.html