Knowledge base

For the sake of simplicity, the SMTP protocol (used for sending e-mails) includes several stages, the first of which consists of the sender announcing who they are via a command called HELO, which should be followed by a valid address.

If you receive errors referring to bogus HELOs, this means that parameters (used by the person attempting to write to you or by the software you are attempting to use) do not comply with the RFC ("Internet regulation"). This avoids the spread of viruses and spam.

For example, we refuse any message coming from a mail server called "server" instead of a complete name such as "server.domain.xyz" ("Fully Qualified Domain Name" or FQDN). An IP address cannot be used to specify the server either. Examples of refused messages:

Remote server's mail service is not 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 name that is not in a canonical form (FQDN):

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

or is on a list of HELOs blacklisted by us:

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

This will always end in error:

250 mta-gw1.infomaniak.ch Hello, pleased to meetyou
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] ...

Actions to be carried out

Check your mail server configuration to ensure that it uses a correct HELO parameter when establishing SMTP connections

References

http://www.faqs.org/rfcs/rfc2821.html (point 4.1.1.1)