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.
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: 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:firstname.lastname@example.org" "P:XXXXXXXXX" "M:LOGIN PLAIN"
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:
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 email@example.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: 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: 005LJPGI030976: to=, ctladdr= (1000/1000), delay=00:00:02, xdelay=00:00:01, mailer=relay, pri=120375, relay=ssl0.ovh.net. [188.8.131.52], 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
2 thoughts on “Configure Sendmail MTA on CentOS 8 to work as SMTP Relay”
Thanks for the tutorial! I will be trying it out soon.
Quick question: if I am hosting two different websites on the same CentOS machine, am I able to use Sendmail as an SMTP relay for both domain names?
this is a great tutorial, thanks