FQDN
A fully qualified domain name: a complete name down to the root, such as mx1.example.com, as opposed to a short relative name like mx1.
What it means
Strictly, an FQDN ends with a dot for the root (mx1.example.com.), though it is usually written without one. RFC 9499 notes the term is used loosely for any complete name.
SMTP expects fully qualified names: RFC 5321 permits only resolvable FQDNs in SMTP, and relay servers must not try to complete partial names. The EHLO name should be the server's FQDN, and for reverse DNS checks it should match the PTR record of the sending address.
In zone files the trailing dot matters: mx1.example.com without it is read relative to the zone and becomes mx1.example.com.example.com.
Example
EHLO mx1.example.comRelated terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 9499: DNS Terminology, section 2rfc-editor.org
- RFC 5321: Simple Mail Transfer Protocol, section 2.3.5rfc-editor.org
- RFC 5321: Simple Mail Transfer Protocol, section 4.1.1.1rfc-editor.org
- RFC 5321: Simple Mail Transfer Protocol, section 5.1rfc-editor.org
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