Hi Jesper, On Tue, 11 Jun 2024 14:10:11 +0100, Jesper Wallin <jes...@ifconfig.se> wrote: > > This is indeed an invalid hostname. A hostname must begin with a digit > or letter. (https://www.rfc-editor.org/rfc/rfc1123#page-13) >
You are wrong. For example, SRV records (RFC 2782) are widely used and have the format _service._proto.name. Next, I checked RFC 5321 section 5.1 and it uses the following wording: When a domain name associated with an MX RR is looked up and the associated data field obtained, the data field of that response MUST contain a domain name. That domain name, when queried, MUST return at least one address record (e.g., A or AAAA RR) that gives the IP address of the SMTP server to which the message should be directed. Any other response, specifically including a value that will return a CNAME record when queried, lies outside the scope of this Standard. The prohibition on labels in the data that resolve to CNAMEs is discussed in more detail in RFC 2181, Section 10.3 [38]. As you can see, it explicitly uses the domain name, not the hostname. This means that the MX record is a domain name, and it can start with an underscore. -- wbr, Kirill
