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How to configure Sendmail to use SMTP relay

Sendmail is a well-known open-source mail transfer agent. We can configure it to use Simple Mail Transfer Protocol relay to send as well as receive emails. In this article, our experts will demonstrate how to configure Sendmail to use SMTP relay.

1. First, we have to install Sendmail on our system. This often depends on the distribution we are currently using. We can use the following command for Ubuntu or Debian-based systems:

   sudo apt-get install sendmail

   The following command has to be sued if we are working on Red Hat or Fedora-based systems:

   sudo yum install sendmail
2. Next, we have to open /etc/mail/sendmail.mc file in a text editor and ensure the following line is not commented:

   dnl define(`SMART_HOST', `smtp.our.provider')dnl

   Remember to replace smtp.our.provider with the FQDN of the SMTP relay server. For instance:

   define(`SMART_HOST', `relay.bobcares.com')dnl
3. Then, we have to regenerate the Sendmail configuration file with the following command:

   /etc/mail/make
4. At this point, we can authentication of required so by SMTP relay. This is done by changing to the mail configuration directory and then running the following command to create authinfo file:

   Create /etc/mail/authinfo
5. Then, we have to open the authinfo file and add a line containing the SMTP FQDN and Credentials as seen below:

   AuthInfo:smtp.domain.com "U:USERNAME" "P:PASSWORD" "M:PLAIN"

   Remember to replace smtp.domain.com, Username, and Password.
6. After that, we have to create a hash map as seen below:

   makemap hash authinfo < authinfo
7. Then, we will add Authinfo to Sendmail config as seen here: vim /etc/mail/sendmail.mcFEATURE(`authinfo’)
8. Finally, we have to regenerate the SendMail Configuration file and then restart the Sendmail service.