Configure Sendmail SSL encryption for sending and receiving email

To configure Sendmail to use SSL encryption for sending and receiving email, you will need to follow these steps:

1. Obtain an SSL certificate for your domain. You will need to obtain an SSL certificate for your domain from a trusted certificate authority (CA). This certificate will be used to encrypt the connection between Sendmail and other mail servers.

2. Install the SSL certificate on the server. Once you have obtained the SSL certificate, you will need to install it on the server where Sendmail is running. The exact steps to do this will depend on your operating system and the method you used to obtain the certificate. For example, on a CentOS-based system, you can use the certutil command to import the certificate into the system certificate database like this:

certutil -A -n "My Certificate" -t "CT,," -d /etc/pki/nssdb -i /path/to/certificate.crt

This will install the SSL certificate on the server.

3. Edit the /etc/mail/sendmail.mc file and add the following lines to enable SSL encryption:

define(`confCACERT_PATH', `/etc/pki/tls/certs')
define(`confCACERT', `/etc/pki/tls/certs/ca-bundle.crt')
define(`confSERVER_CERT', `/path/to/certificate.crt')
define(`confSERVER_KEY', `/path/to/certificate.key')
define(`confCLIENT_CERT', `/path/to/certificate.crt')
define(`confCLIENT_KEY', `/path/to/certificate.key')
DAEMON_OPTIONS(`Port=smtps, Name=TLSMTA, M=s')

These lines will enable SSL encryption for Sendmail and configure it to use the SSL certificate that you installed in step 2.

4. Run the make command to rebuild the sendmail.cf configuration file from the sendmail.mc file. This step is required because the sendmail.mc file is used as a template to generate the sendmail.cf file, which is used by the Sendmail daemon. To rebuild the sendmail.cf file, you can run the following command:

make -C /etc/mail

This will rebuild the sendmail.cf file from the sendmail.mc file.

5. Restart the Sendmail service. After you have made the above changes, you will need to restart the Sendmail service to apply the changes. To restart the service on a CentOS-based system, you can use the service command like this:

service sendmail restart

This will restart the Sendmail service and apply the changes.