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Normally, the reason you have reached this page is because a mail server has sent you a message when it rejected an email from you, or one of your users.
Although email servers can (by RFC) accept connections that have no HELO identifier, most email servers and anti-spam tools will not allow this behaviour. This practice does not supply enough information to safely identify the server and responsible party, if a problem email is received. And almost every mail server offers this capability. This is a simple rule which just insists that the HELO (or EHLO) is sent with every email connection. However, you should ensure that the HELO is also properly formatted. (see valid_helo_domain best practices as well)
In order to ensure that messages are not stopped by this check, make sure the HELO or EHLO is issued by your server or scripts.
An example would be:
220-mail.receiveingserver.com ESMTPSpammers, bulk mailers, and trojans and bots often do not handle the HELO portion correctly, and this makes spam detection easier, and also helps you identify exactly what server was causing a problem. Many email administrators who buy off the shelf mail server software may not correctly set up the HELO, and instead it may default to just a generic identifier, or not be sent. Ensure that you correctly configure your mail server. If you are the one sending the message, and you were blocked with this message, it is most likely that your email provider has not set up the mail server according to best practices, however read the section below.
HELO mail.sendingserver.com
250 mail.receiveingserver.com
MAIL FROM: ....
If your email was blocked, and the link sent you here it is probably not your fault. It is usually the fault of your outgoing email server, however it can be caused by other reasons: