What is a fully qualified domain name and why does the trailing dot matter?
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A fully qualified domain name (FQDN) is the full name of a host, written all the way out, like mail.example.com.. In raw DNS it ends with a trailing dot. That dot signals the name is absolute, not relative. It represents the root of the DNS hierarchy.
You will see it in full DNS output. An MX lookup can come back as 10 mail.example.com., and the dot at the end is normal.
Where it bites is in zone files and DNS panels. Some DNS tools add the trailing dot automatically. Others don't. A missing trailing dot on a fully qualified name can cause lookup failures depending on the resolver, and it is one of the causes of an "invalid MX record" error. The opposite mistake happens too. If your provider adds the dot for you and you add one by hand, you can end up with a doubled dot that breaks resolution. Check your provider's documentation so you know which way yours works.
The full name matters when your server says hello, too. RFC 5321 asks for a fully qualified domain name that actually resolves in DNS in the HELO or EHLO command, not an IP literal, not localhost, and not a bare hostname.
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