Is it possible to secure Mailman on each domain separately?
By default Mailman uses certificate set up in Tools & Settings > SSL/TLS certificates. Mailman does not create individual configuration files for each virtual host. Due to this it is not possible to issue separate certificate for each domain.
Please take part in our product improvement by voting for the corresponding feature suggestion on our UserVoice portal.
Top-voted suggestions are likely to be included in the next versions of Plesk.
Log into Plesk;
Install certificate on Plesk panel in Tools & Settings > SSL/TLS certificates (for example, Let's encrypt);
Make it default:
Note: In this way all mailing lists will be secured by Let's Encrypt certificate, however, due to domain name mismatch (certificate is issued for hostname like example.com and Mailman uses lists.example.com) you will get a warning that certificate validation is failed.