Webmail is not accessible via HTTPS: Connection was reset

Symptoms

• When attempting to access Webmail via HTTPS, it fails to open:

  CONFIG_TEXT: ERR_CONNECTION_RESET

• In IIS > Sites > MailEnable Webmail > Bindings..., the bindings are only for 80 port:
  

Cause

Only one type of bindings (either HTTP or HTTPS) can be enabled for webmail sites. By default, all webmail on Windows works over HTTP only.

Resolution

It works this way due to certain limitations of IIS. Its performance degrades when the server has lots of domains hence a large configuration file applicationHost.config.
The bigger this file gets, the slower IIS becomes.
Starting from 12.5, Plesk was built with avoiding unnecessary additions to this file in mind.
Considering this performance aspect, during the implementation of "HTTPS support for webmail" it was decided not to add 2x bindings.
So, only one type of bindings (either HTTP or HTTPS) for webmail site was implemented.

To enable HTTPS for webmail:

1. Log into Plesk;
2. Secure the domain with Let's Encrypt;
3. Navigate to Subscriptions > example.com > Mail > example.com > Mail Settings;
4. Choose the newly created certificate in 'SSL/TLS certificate for webmail':
   

Note: To enable redirect from HTTP to HTTPS, refer to this guide.