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

[jindam, vani]

* emacs on debian bullseye on userland (1)
* the documentation(2) says about 501 helo command.
* complete error:
Debugger entered--Lisp error: (error "Sending failed: 504 5.5.2 
<localhost>: Helo command rejected: need fully-qualified hostname")
  signal(error ("Sending failed: 504 5.5.2 <localhost>: Helo command 
rejected: need fully-qualified hostname"))
  error("Sending failed: %s" "504 5.5.2 <localhost>: Helo command 
rejected: need fully-qualified hostname")

* my hostname settings:

$ hostname
localhost

$ cat /etc/hosts
127.0.0.1 localhost

$ cat /etc/hostname
localhost

* my configs:
** .gnus
(setq gnus-select-method '(nnnil ""))

(add-to-list 'gnus-secondary-select-methods
             '(nnmaildir ""
              (directory "/home/jin/.Maildir/")
              (get-new-mail t)
              (directory-files nnheader-directory-files-safe)))

(setq mail-user-agent 'gnus-user-agent)
(setq read-mail-command 'gnus)

(setq mail-sources
                  '((pop :server "disroot.org"
                         :user "jindam.vani@disroot.org"
                         :password "password")))

(setq message-send-mail-function 'smtpmail-send-it)
(setq smtpmail-default-smtp-server "disroot.org")
(setq smtpmail-smtp-service 587)

** .msmtprc
(output of $ msmtp --configure jindam.vani@disroot.org
# - copy this to your configuration file /home/jin/.msmtprc
# - add your password to the key ring:
#   secret-tool store --label=msmtp host disroot.org service smtp
 user jindam.vani
account jindam.vani@disroot.org
host disroot.org
port 587
tls on
tls_starttls on
auth on
user jindam.vani
from jindam.vani@disroot.org

** getmailrc
[options]
delete = True
read_all = true

[retriever]
type = SimplePOP3SSLRetriever
server = disroot.org
username = jindam.vani@disroot.org
password = password

[destination]
type = Maildir
path = ~/.Maildir/INBOX/

how do i resolve?

(1) https://github.com/CypherpunkArmory/UserLAnd
(2) 
https://www.gnu.org/software/emacs/manual/html_node/smtpmail/Server-workarounds.html#index-smtpmail_002dlocal_002ddomain

regards,
jindam, vani

--
toots: @jindam_vani@c.im
others: en.wikipedia.org/wik//User:Jindam_vani