Configure Sendmail MTA on CentOS 8 to work as SMTP Relay

Sendmail is easy to implement, lightweight electronic Mail Transport Agent (MTA), which enables you to automate the process of sending e-mail messages from your Linux host. It can play a significant role as a background mechanism for processing outbound emails from monitoring software to deliver status notifications to the system administrators or send diagnostic information at a specified time schedule.
In this article, I present how to install and configure Sendmail on CentOS 8 to work as SMTP Relay for outbound traffic to the OVH mail server.

0. Prerequisites

Sendmail daemon requires configured hostname for proper operation.
Set hostname for your host (if not already set):

[root@chronos ~]# hostnamectl set-hostname chronos

Include your hostname in the /etc/hosts file:

[root@chronos ~]# cat /etc/hosts
127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4 chronos
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6

1. Install sendmail service

Install sendmail and corresponding RPM packages:
– sendmail-cf package includes configuration files required to generate sendmail.cf configuration file.
– cyrus-sasl-plain package contains the Cyrus SASL plugins which support PLAIN and LOGIN authentication.

[root@chronos ~]# dnf install sendmail sendmail-cf cyrus-sasl-plain

Note: If you don’t have cyrus-sasl-plain package installed and your SMTP relay host requires authentication, you might encounter the following error in sendmail log:

Jan 05 00:37:07 chronos sendmail[758]: 004NXT6g000357: AUTH=client, available mechanisms do not fulfill requirements

2. Generate authentication information for SMTP relay

Create authinfo text file containing SMTP relay authentication info:

[root@chronos ~]# touch /etc/mail/authinfo

Edit the file, provide SMTP Relay FQDN and authentication credencials:

AuthInfo:ssl0.ovh.net "U:root" "I:admin@tuxfixer.com" "P:XXXXXXXXX" "M:LOGIN PLAIN"

where:
I: account username at your ISP
P: account password at your ISP

Generate the authentication database, based on authinfo text file (this will overwrite existing /etc/mail/authinfo.db file):

[root@chronos ~]# cd /etc/mail/
[root@chronos mail]# makemap hash authinfo < authinfo

Change permissions of both authinfo and authinfo.db files (read/write access for root only):

[root@chronos ~]# chmod 600 /etc/mail/authinfo*

3. Modify sendmail configuration source file

Edit sendmail.mc source file (DO NOT edit resultant sendmail.cf configuration file):

[root@chronos ~]# vim /etc/mail/sendmail.mc

Uncomment and modify SMART_HOST related line:

define(`SMART_HOST', `ssl0.ovh.net')dnl

Add authentication database file related line to let the sendmail daemon find authentication credencials:

FEATURE(`authinfo', `hash -o /etc/mail/authinfo.db')dnl

Regenerate /etc/mail/sendmail.cf file:

[root@chronos ~]# /etc/mail/make

4. Launch the sendmail service

Start and enable sendmail daemon:

[root@chronos ~]# systemctl start sendmail
[root@chronos ~]# systemctl enable sendmail

Verify, if sendmail started successfully:

[root@chronos ~]# systemctl status sendmail

5. Test the e-mail delivery via SMTP Relay

Now it’s time to test sendmail functionality and send test e-mail from Linux command line.
Prepare example e-mail body:

[root@chronos ~]# echo "Subject: Mail Delivery Test" > /tmp/mail.txt
[root@chronos ~]# echo "This is the example e-mail body" >> /tmp/mail.txt

Send the test e-mail using sendmail command:

[root@chronos ~]# sendmail -v admin@tuxfixer.com < /tmp/mail.txt

I am using -v parameter for increased verbosity (to see the whole communication with SMTP relay server).

You can also check systemd journal to see if the e-mail was sent successfully:

[root@chronos ~]# journalctl -f -u sendmail
...
Jan 05 22:19:26 chronos sendmail[30978]: STARTTLS=client, relay=ssl0.ovh.net., version=TLSv1.2, verify=OK, cipher=ECDHE-RSA-AES256-SHA, bits=256/256
Jan 05 22:19:27 chronos sendmail[30978]: 005LJPGI030976: to=, ctladdr= (1000/1000), delay=00:00:02, xdelay=00:00:01, mailer=relay, pri=120375, relay=ssl0.ovh.net. [193.70.18.144], dsn=2.0.0, stat=Sent (Ok: queued as 04209DE605A0)

Finally, check the recipient’s Inbox (or SPAM box), the test e-mail should be delivered successfully.

Additionally, to see mail queue, execute the command:

[root@chronos ~]# mailq