You may receive a "451" error message from (mt) Media Temple's mail servers similar to the one below.
Temporarily Unavailable - GRID service has exceeded sending limit for email containing spam-like content.
This means that you have been temporarily flagged or metered for potential spamming. Specifically, it means that you have sent a number of messages that have received a score of 5 or higher on our spam filtering software. As per our Acceptable Usage Policy, our system could not allow your message to be sent. This only applies to messages that are ranked with a high spam score within a specified time period (one hour). All messages with "normal" scores are sent without issue.
Spam is a large problem on the Internet and can lead to mail servers becoming blocked for future sending. This is especially painful in a shared environment, because one spammer can affect many innocent users. It is our responsibility as a mail service provider to take measures to prevent spam from originating from our network.
While it is in everyone's best interest to stop spam, we do not want to inconvenience legitimate users who may have inadvertently triggered our scanners. To determine why your email attempt resulted in this error, and to prevent future emails from being blocked, please follow the troubleshooting steps shown below.
Often, eliminating spam-like characteristics from your email will stop it from getting blocked. Keep in mind that it is usually not one single element that is contributing to your high spam score - you may have to look into two or three of these factors.
Alternately, if the IP address from which you are sending has been identified as a source of spam, you may have to use an alternate outgoing server until that gets resolved.
Sending server reputation
If your sending server has recently sent out a lot of spam, all messages coming from that server may be blocked, even if they are not spam. You can use a third-party website such as MX Toolbox to check your IP address against many different block lists at once. There are several free websites available for this kind of check.
HTML content and signatures
Messages with HTML content in the body or signature are more likely to be identified as spam.
Messages with links to external web pages are more likely to be identified as spam.
ALL CAPS and Misspellings
Avoid using ALL CAPITALS in your subject line or the body of your message. Run your message through spell-checking before sending it.
If your message contains reference to "typical" spam subjects - pharmaceuticals, for example - you may be flagged for these words.
"Spoofing" occurs when a spammer sends out emails under your domain name or email address, even though they are not actually coming from your server. You can reduce this behavior by allowing other mail servers to verify the server your domain should be using for sending mail using an SPF record in your zone file. For more information, please see: How can I create an SPF record for my domain?
Sender callback verification and HELO string resolution
If your sending mail server does not produce a valid response to the sender callback verification HELO string, you can request that your (mt) Media Temple contact add you to their list of allowed senders.
See How do I set up spam filtering for my server? for adding users to your allow list.
This will allow the (mt) Media Temple mail server to bypass the HELO string and sender callback verification.
You can run your email through a third-party spam score checking service. There are several free and paid services that perform this checking available online.