a. Fetch your certificate

1.You will receive an email when your certificate is issued.
2.Select the 'Standard certificate' format option to download your issued certificate.
3.Copy and Paste your certificate (first certificate) to a text editor (Notepad) and save the file as yourdomain.crt
4. Copy and Paste the Intermediate Certificate (second certificate) to a text editor (Notepad) and save the file as intermediate.crt

b. Installing your Certificate on Postfix

1. Locate and open the Postfix config file main.cf .

2. Save yourdomain.crt to the same directory as the yourdomain.key file which would have been created when you originally created your CSR. If you are importing an existing certificate, ensure you also import the yourdomain.key file.

3. Save the intermediate.crt to the same directory as yourdomain.crt and yourdomain.crt .

4. In the the Postfix TLS settings section ensure the following directives and file locations are present. You will need to add them if they are not present or if they are different:

smtpd_tls_cert_file = /some/path/yourdomain.crt
smtpd_tls_key_file = /some/path/yourdomain.key
smtpd_tls_CAfile = /some/path/intermediate.crt

The following settings may be different per server, however ensure that your setup is similar to the below examples:

smtpd_use_tls = yes
smtpd_tls_auth_only = yes
smtpd_tls_session_cache_timeout = 3600s