The Let’s Encrypt certificates work fine for mail servers ( assuming you want them for POP3, IMAP, SMTP rather than encryption of emails themselves. Why do you say the certifcate does not work ?
If you, somehow, get a cert just for mail.higain.ca then yes, when using that certificate, it will only work for that hostname.
Also, I am not aware of any many mail server with don’t have SNI capabilities (EXIM does have SNI support), so if you want multiple hostnames for your single mail server with just one certificate, you must add all those hostnames to that one certificate.
This partially depends on what you use for your mail service, and what version it is.
The easiest is often to use the “server name” to connect to general server services like email - and then use the appropriate certificate for that “server name” which needs to be a FQDN of course.
Many of the recent mail services can use different certificates for different domains, but this is more complex to set up.
The questions here are more about server setup and email server setup rather than certificates ( which this forum is about). Not a complaint - but you might find that detailed help on configuring plesk for email setup, you might find quicker answers from a plesk forum.