Ubuntu 23.04
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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

apt -y install mailutils
# set environment variables to use Maildir

echo 'export MAIL=$HOME/Maildir/' >> /etc/profile.d/mail.sh
# add an OS user [ubuntu]

adduser ubuntu

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

mail ubuntu@localhost

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

# to finish messages, push [Ctrl + D] key

# see received emails


"/home/ubuntu/Maildir/": 1 message 1 new
>N   1 ubuntu             Tue May 17 00:28  13/448   Test Mail #1

# input the number you'd like to see an email
? 1
Return-Path: <ubuntu@mail.srv.world>
X-Original-To: ubuntu@localhost
Delivered-To: ubuntu@localhost
Received: by mail.srv.world (Postfix, from userid 1000)
        id 47C4948C65; Fri, 28 Apr 2023 11:04:34 +0000 (UTC)
To: <ubuntu@localhost>
Subject: Test Mail #1
User-Agent: mail (GNU Mailutils 3.15)
Date: Fri, 28 Apr 2023 11:04:34 +0000
Message-Id: <20230428110434.47C4948C65@mail.srv.world>
From: ubuntu <ubuntu@mail.srv.world>

This is the first mail.

# to quit, input [q]
? q
Saved 1 message in /home/ubuntu/mbox
Held 0 messages in /home/ubuntu/Maildir/
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