Is it possible to secure the mail server
mail.example.com with Let’s Encrypt SSL certificate when the A record for
example.com is pointing to another server?
Currently, such a functionality is not implemented in Plesk yet.
Consider voting for this feature on the UserVoice portal. The top-ranked suggestions are likely to be included in the next versions of Plesk.
Alternatively, issue a certificate from another authority and secure mail using it:
How to install an SSL certificate for a domain in Plesk (Let’s Encrypt / other certificate authorities)
- Create a subdomain mail.example.com
- Go to Domains > mail.example.com > SSL/TLS Certificates
- Issue a new certificate
- Go to Domains > example.com > Mail Settings
- Assign the certificate for mail.example.com