FYI - PI-008149
The SMTP Server settings do not appear to work if the email address contains an underscore, or any special characters I suspect.
I was trying to test this config with firstname.lastname@example.org and would continue to get the same error message in the Jamf server logs.
javax.mail.AuthenticationFailedException: 535-5.7.8 Username and Password not accepted. Learn more at 535 5.7.8 https://support.google.com/mail/?p=BadCredentials
The support article noted there suggests enabling "Less secure app mode" on the account itself, which was done.
I managed to successfully configure the SMTP Server settings up by using email@example.com instead.
We are using G Suite for our email provider.
Thanks @dlondon, I just heard back from Jamf Support and they have raised a Product Issue for this. Post has been updated.
It's not yet listed in https://www.jamf.com/jamf-nation/my/assets/jamf-pro/known-issues but I have been assured that it will be soon.