CentOS Stream 9
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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 s-nail
# set environment variables to use Maildir

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

[root@mail ~]#
useradd cent

[root@mail ~]#
passwd cent

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

[cent@mail ~]$
mail cent@localhost

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

# to finish messages, push Ctrl + D key
(Preliminary) Envelope contains:
To: cent@localhost
Subject: Test Mail#1
Send this message [yes/no, empty: recompose]? yes

# see received emails

[cent@mail ~]$

s-nail version v14.9.22.  Type `?' for help
/home/cent/Maildir: 1 message 1 new
►N  1 cent@srv.world        2022-03-18 19:23   14/405   "Test Mail#1"

# input a number you'd like to see an email
& 1
[-- Message  1 -- 14 lines, 405 bytes --]:
Date: Fri, 18 Mar 2022 19:23:33 +0900
To: cent@localhost
Subject: Test Mail#1
Message-Id: <20220318022333.B1738EA09@mail.srv.world>
From: cent@srv.world

This is the first mail.

# to quit, input [q]
& q
Matched Content