Cannot assign certificate to mail when installing Let's Encrypt certificate in Plesk

Symptoms

Assign the certificate to the mail domain option is missing when installing Let's Encrypt certificate in Plesk.

Cause

Courier is used on the server, which does not support SNI.

Resolution

Warning: This process takes down mail services for up to 30 minutes, so please schedule this for the appropriate maintenance time.

1. Log into Plesk
2. Go to Tools and settings > Updates page and choose IMAP server as Dovecot. This will uninstall Courier as IMAP server.
3. Wait for the installation process to finish.

Additional information:

How to assign a SSL certificate per domain to secure the mail server in Plesk (SNI support)?