Let's Encrypt certificate or certificate from other certificate authorities is installed for domain or webmail. Where is this certificate located on the server?
All certificates used by domain or webmail are located in
Information about what certificate file matches domain name can be found in appropriate configuration file:
# grep SSLCertificateFile /etc/httpd/conf/plesk.conf.d/webmails/example.com_webmail.conf
# grep SSLCertificateFile /etc/httpd/conf/plesk.conf.d/vhosts/example.com.conf