SYMPTOMS

When sending to a remote mail server the receiving server may employ greylisting checks and not accept the first send and return an SMTP error code of:

"451 This server employs greylisting as a means of reducing spam. Please resend e-mail shortly"

CAUSE

Remote mail server employs greylisting.

WORKAROUND

There is no workaround to bypass greylisting as it is a common for mail servers to employ greylisting checks and delay the first send attempt as a method to reduce inbound spam.  MailEnable will queue the outbound message that has been delayed by the remote mail server and retry at a later time. Once the receiving server accepts the message it will whitelist the IP address and senders email address for a period of time so that future attempts from the same IP address and senders email address will bypass the greylisting checks and not be delayed again (this setting is dependent on the remote mail servers greylisting configuration settings).

The remote mail server's greylisting will be responsible for when the SMTP service accepts a connection from the same IP address and sender's email address. MailEnables first retry attempt may not cater for the retry interval and may delay the second send. Therefore you may need to tweak the retry attempts within MailEnable to help cater for the remote servers greylisting retry intervals. The SMTP "Delivery" retry attempts are configured within the SMTP properties under the "Delivery" tab (http://www.mailenable.com/documentation/8.0/Enterprise/Delivery.html).

To prevent MailEnable from generating persistent bounce messages for every delayed attempt you may need to tweak the SMTP "Delay Notification" settings. Set the "Delay notifications" settings so that only one notification message is sent back to the sender. It is also recommended to set the "Delay notification" intervals to occur after the second retry attempt. The reason for this is to avoid a notification after the first delay as its common for most mail servers to retry after some minutes thus avoiding the need to send a delay notification.  

REFERENCES

http://projects.puremagic.com/greylisting/

https://en.wikipedia.org/wiki/Greylisting