This is a basic installation only,

You can watch the tutorial on YouTube video below for better understanding:

Tested on RockyLinux 9.3, should be compatible to other RHEL / CentOS distributions

What you need:

• a Gmail account with 2-Step Verification enabled,
• an app password created in the Gmail account.

LET'S BEGIN:

run all commands as sudoers

sudo bash

install the packages needed

sudo dnf -y install cyrus-sasl cyrus-sasl-lib cyrus-sasl-plain postfix s-nail

save the gmail account app password to a file

nano /etc/postfix/sasl_passwd

write your Gmail Account name and Gmail app password you create in this file with this format

[smtp.gmail.com]:587 [email protected]:gmail.app.password

save the file and exit nano

set the password file to postfix database

postmap hash:/etc/postfix/sasl_passwd

remove the password file

rm -f /etc/postfix/sasl_passwd

edit the main configuration of postfix

nano /etc/postfix/main.cf

append this to the file

relayhost = [smtp.gmail.com]:587
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_security_options = noanonymous
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_tls_security_level = secure
smtp_tls_mandatory_protocols = TLSv1.3
smtp_tls_mandatory_ciphers = high
smtp_tls_secure_cert_match = nexthop
smtp_address_preference = ipv4

save the file and exit nano

enable postfix to start at system startup

systemctl enable postfix

start (or restart) postfix right now

systemctl restart postfix

send your first email

echo "This is the email body " | mail -s "Email subject" -r "[email protected]" [email protected]

The previous steps can only send email limited from within it self machiche,

Do the next step so other machines can also use this SMTP server

What you need:

• one or more IP Addresses of the other machines to be allowed, for example refer to https://www.postfix.org/BASIC_CONFIGURATION_README.html#relay_from
• one or more username and password on this SMTP Server for sending email from the other machine

let's begin, start by editing the postfix main configuration again,

edit and append the following

nano -w /etc/postfix/main.cf

inet_interfaces = all
# !!!! PLEASE ENTER _one or more IP Addresses_ of the other machines to be allowed below
mynetworks = 192.168.1.0/24
smtpd_relay_restrictions=permit_mynetworks, reject
smtpd_sender_restrictions = permit_mynetworks, reject
smtpd_tls_auth_only = no
smtpd_sasl_auth_enable =   yes
smtpd_sasl_type = cyrus
local_recipient_maps =
unknown_local_recipient_reject_code = 550
smtpd_error_sleep_time = 1s
smtpd_soft_error_limit = 10
smtpd_hard_error_limit = 20

save the file and exit nano

then edit master.cf file, replace the first smtp inet n - n - - smtpd line, edit with the following

nano -w /etc/postfix/master.cf

smtp   inet n   -   n   - - smtpd
      -o smtpd_sasl_auth_enable=yes
      -o smtpd_reject_unlisted_sender=yes
      -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
      -o broken_sasl_auth_clients=yes

save the file and exit nano

enable the authentication daemon at startup

systemctl enable saslauthd

start the authentication daemon right now

systemctl start saslauthd

restart postfix right now

systemctl restart postfix

open the machine firewall at port 25 to listen for SMTP request from other machine

firewall-cmd --add-port=25/tcp --permanent && firewall-cmd --reload

DONE, now you can try SMTP from other machines with an assigned username and password from this SMTP Server !