Plesk allows you to add the free Let's Encrypt SSL Certificate for your domains. Since Let's Encrypt provides wildcard SSL for your domains so it will also cover your webmail.
If you have purchased your SSL Certificate from a third-party host, you can refer to the steps below to install it. It is straightforward steps without any complexity.
Please refer to the steps below to install a third-party SSL Certificate on Plesk webmail.
Step 1: Log in to your Plesk Panel.
Step 2: At Websites & Domains, click on the SSL/TLS Certificates.
Step 3: Click on the Add SSL/TLS Certificate button.
Step 4: Enter the name of your SSL certificate. You can choose any of the names here.
Step 5: Upload your SSL Certificate and Private key, and click on the Upload Certificate button.
Step 6: Check the box of SSL Certificate and click on the secure webmail.
Step 7: You will see the message below that Webmail is secured with the SSL Certificate. Now you can access your webmail using https.
