add posibility to secure webmail with Let's Encrypt certificate for the subdomain with 'No web hosting' type
The issued certificate should be also available for selecting in Domains > subdomain.name > Mail Settings > SSL/TLS certificate for mail

Thank you for your input. We will consider this functionality in upcoming releases if it is popular. Everyone, please continue voting for this feature if you consider it important.
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IG
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Plesk Tech Support commented
Currently, Plesk implements SNI so that each mail server can be protected by an individual SSL certificate. However, this only works if the domain has hosting. Mail-only domains cannot be secured with individual SSL certificates and need to rely on the server-wide certificate instead.
There should be an option to issue and assign individual SSL certificates to protect mail-only domains.