The most common reason not to be able to join the domain in System Setup > Authentication when using NTLM is not using the actual NT DOMAIN, but the email domain.

- Typically the NT domain is just one word, no ".com", no ".net", no ".local"

The most common error is:  Error: Unable to join domain (-4)

- The NT Domain should look something like this when you add it:

Also, for the login, this is usually just the user name, and not the email address or domain/username formats.