Linux Configure Sendmail as SMTP Mail Client ( submission MTA )

Sendmail is a popular mail transfer agent (MTA) that is used to send and receive email on Linux systems. It can be configured as a Simple Mail Transfer Protocol (SMTP) client (also known as a submission MTA) to send email through an SMTP server.

To configure Sendmail as an SMTP client on a Linux system, you will need to edit the Sendmail configuration files and specify the SMTP server that you want to use. The specific steps and configuration options will vary depending on your Linux distribution and version.

Here are the general steps for configuring Sendmail as an SMTP client on a Linux system:

1. Install Sendmail. If Sendmail is not already installed on your system, you can install it using the package manager for your Linux distribution (e.g., apt, yum, pacman).

For example, on a Debian-based system, you can use the following command to install Sendmail:

$ sudo apt-get install sendmail

2. Edit the sendmail.mc configuration file. The sendmail.mc file is a configuration file that is used to generate the sendmail.cf configuration file for Sendmail. You will need to edit the sendmail.mc file to specify the SMTP server that you want to use.

To specify the SMTP server, you will need to add a define line to the sendmail.mc file. The define line should have the following format:

define(`SMART_HOST', `[SMTP server]')

Replace [SMTP server] with the hostname or IP address of the SMTP server that you want to use.

For example, to use the SMTP server at smtp.example.com, you would add the following line to the sendmail.mc file:

define(`SMART_HOST', `smtp.example.com')

3. Generate the sendmail.cf configuration file. After you have edited the sendmail.mc file, you will need to generate the sendmail.cf configuration file. You can use the m4 command to generate the sendmail.cf file from the sendmail.mc file.

For example, to generate the sendmail.cf file, you can use the following command:

$ sudo m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

4. Restart the Sendmail service. After you have generated the sendmail.cf configuration file, you will need to restart the Sendmail service to apply the changes. You can use the service or systemctl command to restart the Sendmail service, depending on your Linux distribution.

For example, on a Debian-based system, you can use the following command to restart the Sendmail service:

$ sudo service sendmail restart

On a Red Hat-based system, you can use the following command to restart the Sendmail service:

$ sudo systemctl restart sendmail

5. Test the configuration. You can test the Sendmail configuration by sending a test email using the sendmail command.

For example, to send a test email to user@example.com, you can use the following command:

/usr/sbin/sendmail user@domain.com < /filecontent/to/send