Re: smtpmail-send-it: Sending failed: 504 5.5.2 <localhost>: Helo command rejected: need fully-qualified hostname

[Björn Bidar]

Do you really need to add the credentials here? Try without them so
smptmail will try to use auth-source to retrieve the credentials.

These are my smtpmail settings:
      (use-package smtpmail
        :config
        (setq smtpmail-default-smtp-server "smtp.server.de"
              smtpmail-smtp-server "smtp.server.de"
              smtpmail-smtp-service "465"
              smtpmail-stream-type 'ssl
              smtpmail-servers-requiring-authorization ".*"))

For me these work like this, smtpmail will try to retrieve the
localdomain and sendto-domain by itself.

If you add either to the hosts file they should be fine.

Br,

Björn