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 thawte certificate to a text editor (Notepad) and save the file as cert.pem

b. Install the certificate for Qpopper:

Assuming this signed request is in a file called signed_req.pem, concatenate it to the private key generated earlier:
cat signed_req.pem >> /etc/mail/certs/cert.pem
Create a configuration file for Qpopper. You can locate this file anywhere you choose. For example, /etc/mail/pop/qpopper.config. Put the paths to the private key and signed certificate in this file, and enable either alternate-port or STLS. For example, using the above file names and STLS:
set tls-support = stls
set tls-server-cert-file = /etc/mail/certs/cert.pem

Use the -f config-file-path command-line option to tell Qpopper to read the configuration file.