You can secure plesk,add host name as own hosting space,than log in to and add lets encrypt SSL as you do for any domain.After that go to tool/server setting and select that SSL as plesk default ssl
Just to say ...you can do it even more professional..with just one more IP.Set that IP as dedicated for hostname..than add .htaccess rule to redirect IP to hostname...just in case someone go through Ip...
Plesk, yes, webmail, no. You can't use let's encrypt to secure webmail. Its a nice idea, but technically impossible Here is why:
Let's Encrypt don't create wildcards, that not a Plesk issue, Let's encrypt simply don't dont do wildcards. you need a wildcard cert to use the "also secure webmail" feature.
Lets take domain example.com as example
The certificate generated bij Let's Encrypt is for example.com and www.example.com.
The webmail url is always webmail.example.com, the certificate for example.com and www.example.com will give a name mismatch error on subdomain webmail.example.com. There is no way to create a Let's Encrypt certificate for a webmail subdomain because the .well-known can not be read by Let's encrypt.
The only way to use the "also use certificate for webmail" feature, is when you buy a wildcard certificate.
Maybe its technically possible, but not in the real world because this must be run as root, even a user with full ssh access (/bin/bash) can't run this.
-bash-4.1$ plesk bin extension --exec letsencrypt -d linulexhosting.nl
Could not open Repository at "/etc/sw/keys": Cannot open file
Error: must run as root.
exit status 1
The whole point of plesk and something like the Let's Encrypt add-on is that users can do it them self without interference of the sys-admin.