Even though Plesk allow us to redirect automatically the main domain with “SEO-safe 301 permanent redirect from HTTP to HTTPS” in the control panel but isn’t possible to do the same thing with the subdomain “webmail.domain.com”.
So When installing the let’s encrypt certificate for your domain you have an option to also use it for your plesk webmail.
But it won’t work right away even after you have added the SSL certificate so we have to force it. If you desire a permanent “HTTPS” – usage, you could ADD at “/usr/share/psa-roundcube/config/config.inc.php” (why at “config.inc.php”? Because Plesk might update/upgrade the Plesk – roundcube – package from time to time and you might want your unique modifications even after updates/upgrades! ):
So add the below code to the this file /usr/share/psa-roundcube/config/config.inc.php
$config['force_https'] = true;
If you have any issue please use the comment form below.