Fedora 34
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Mail Server : Add Mail User Accounts
Add Mail User Accounts to use Mail Service.
This example is for the case you use OS user accounts.
Configure basic Postfix settings, and basic Dovecot settings first.
[1] To use OS user accounts, that's only adding OS user like follows.
# install mail client program

[root@mail ~]#
dnf -y install mailx
# set environment variables to use Maildir

[root@mail ~]#
echo 'export MAIL=$HOME/Maildir' >> /etc/profile.d/mail.sh
# add a user [fedora]

[root@mail ~]#
useradd fedora

[root@mail ~]#
passwd fedora

[2] Login as a user added in [1] and try to send an email.
# send to myself [mail (username)@(hostname)]

[fedora@mail ~]$
mail fedora@localhost

# input subject
Subject: Test Mail#1
# input messages
This is the first mail.

# to finish messages, input period(.) and Enter
# see received emails

[fedora@mail ~]$

Heirloom Mail version 12.5 7/5/10.  Type ? for help.
"/home/fedora/Maildir": 1 message 1 new
>N  1 fedora@srv.world      Thu May 20 23:18  17/521   "Test Mail#1"

# input a number you'd like to see an email
& 1
Message  1:
From fedora@srv.world Thu May 20 23:18:45 2021
Return-Path: <fedora@srv.world>
X-Original-To: fedora@localhost
Delivered-To: fedora@localhost
Date: Thu, 20 May 2021 23:18:45 -0600
To: fedora@localhost
Subject: Test Mail#1
User-Agent: Heirloom mailx 12.5 7/5/10
Content-Type: text/plain; charset=us-ascii
From: fedora@srv.world
Status: R

This is the first mail.

# to quit, input [q]
& q
Held 1 message in /home/fedora/Maildir
Matched Content