This guide details the 'bogus HELO' email error to help you avoid it.

When and why does this error occur?

To put it simply, the SMTP protocol (used to send emails) has several phases, the first of which involves the sender announcing who they are via a command called HELO, which must be followed by a valid address.

If you receive errors mentioning bogus HELO, it means that parameters (used by the person trying to write to you or by the software you are trying to use) are not compliant with the RFC (point 4.1.1.1). This prevents the spread of viruses and spam.

Infomaniak, for example, refuses any message from a mail server that would be named 'server' instead of having a full name like 'server.domain.xyz' (= FQDN, 'Fully Qualified Domain Name' or 'fully qualified hostname'). An IP address cannot simply define the server. Examples of rejection:

The mail service of the remote server has not been configured with a valid FQDN:

220 mta-gw1.infomaniak.ch ESMTP Infomaniak Network Mail Servers;HELO 127.0.0.1 

or the mail server is not configured to use a real FQDN:

220 mta-gw1.infomaniak.ch ESMTP Infomaniak Network Mail Servers;HELO [127.0.0.1] 

or the mail server is not configured to use a real FQDN:

220 mta-gw1.infomaniak.ch ESMTP Infomaniak Network Mail Servers;HELO localhost 

or the mail server returns a host that is not in canonical form (FQDN):

220 mta-gw1.infomaniak.ch ESMTP Infomaniak Network Mail Servers;HELO www 

or it is on a list of banned HELOs at Infomaniak:

220 mta-gw1.infomaniak.ch ESMTP Infomaniak Network Mail Servers;HELO fakedomain.xyz

This will always result in an error:

250 mta-gw1.infomaniak.ch Hello, pleased to meet you
MAIL FROM: user123@infomaniak.ch
250 2.1.0 user123@infomaniak.ch... Sender ok
RCPT TO: user123@infomaniak.ch
550 5.7.1 user123@infomaniak.ch... Access denied - bogus HELO [1] …

Check the configuration of your mail server so that it uses a correct HELO parameter when establishing the SMTP connection.