Security Certificate for Apple mail server

I need to know what kind of security certificate is needed to send emails to an Apple mail server. (class of certificate, information attested by the certificate)

Background:

I'm trying to setup alert emails from my Zyxel firewall appliance to my email account. The Zyxel has a rudimentary outgoing mail client that needs to be connected to a mail server. Apple mail is my primary email.

I've generated a third-party app password for my Appleid, set the firewall to use TLS, Start TLS and SMTP authentication with my my appleid and the 3rd party password (all these seem to be necessary and working), but now I'm getting a log message that authentication fails because of a self signed certificate in the chain. I note that default certificate on the firewall appliance is self signed, that's likely the problem, easy enough to upload one. The default certificate simply refers to the device ModelNumber_Serial number. Before I spend money to get a certificate traceable to a CA, I need to check if more information is needed than the ModelNumber_Serial number.

Has anyone tried this?