ERROR: 551 Mailhost is on our global blacklist
Posted by on 09 June 2014 12:07 PM
ERROR: 551 Mailhost is on our global blacklist

A person trying to e-mail my organization gets the following error message:

551 Mailhost is on our global blacklist (Mode: normal)

What does this mean?

The e-mail is being sent from an IP address that is listed on a rolling black list (RBL) controlled by McAfee SaaS. This RBL is designed to prevent denial of service (DoS) and directory harvesting attacks (DHAs). DHAs are used by spammers to obtain a list of valid e-mail addresses at a domain, by way of a brute force guessing attack. Suspected attackers are blocked to protect our customers.

IPs that send mostly spam are blocked, as are IPs sending to large numbers of invalid recipients (indicating a DHA).


After an IP address is added to this list, McAfee will continue to monitor the traffic from this source for up to 4 hours. If we see normalized traffic in this timeframe, the IP will be removed from the list.

It is not possible to allow list or whitelist the IPs that are blocked on this blacklist. We recommend that the sender attempt to resend the message 2 - 4 hours after the initial failure.

If you are a customer and wish to opt out of this blacklist, please contact Excel Micro Support and request an opt-out form. T
his is not recommended as it can allow unwanted traffic to be delivered to your users.

If you are not a customer and require assistance, please contact the mail administrator for the organization you are sending to.


The full bounce message will appear similar to the text below. The error message is "551 Mailhost is on our global blacklist."

> ----- The following addresses had permanent fatal errors -----
> (reason: 551 Mailhost is on our global blacklist (Mode: normal))
> ----- Transcript of session follows -----
>... while talking to
>>>> DATA
><<< 551 Mailhost is on our global blacklist (Mode: normal)

ERROR: This domain name does not match domain registered in the license key file (, allowed domains:,localhost, please change the product path to match the domain under Admin CP > Settings > General Settings
This product will not work properly unless untill that value is changed.

For more information please contact Kayako support at