smtp_domain / gitlab configuration

What is the smtp_domain in the GitLab configuration? There is also a smtp_address and smtp_user_name; so what would you put in the “domain” field?

Contrary to what the examples on GitLab Omnibus SMTP lead you to believe: smtp_domain is the HELO/EHLO domain; i.e. your hostname.

RFC 5321 has this to say about the HELO/EHLO parameter:

   o  The domain name given in the EHLO command MUST be either a primary
      host name (a domain name that resolves to an address RR) or, if
      the host has no name, an address literal, as described in
      Section 4.1.3 and discussed further in the EHLO discussion of
      Section 4.1.4.

So, the term [smtp_]helo_hostname would've been a lot more appropriate.