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 for sending emails) involves several phases, the first of which requires the sender to announce their identity using a command called HELO, which must be followed by a valid address.

If you receive errors mentioning bogus HELO, it is because 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 (section 4.1.1.1). This prevents the spread of viruses and spam.

Infomaniak, for example, rejects any message coming 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 either. Examples of rejection:

The remote mail 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 proper 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 proper 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 to ensure it uses a correct HELO parameter when establishing the SMTP connection.