I just wanted to add, with Modern Authentication enabled for emailing, I recommend a non-template email test such as the following, to verify that Modern Auth is enabled and working correctly:
User Notes report
1. Click on Administration > Reports > System > Users
2. In the User Reports window that opens, click 'New...', unless you already have a report option available for use. If you do, jump to step #6.
3. In the report options window, name the option and then choose a range of user(s) to put into the report and click Insert. Save changes.
4. Bring up this same report option created and click the 'Email Options' button at the bottom right-hand side of the window.
**NOTE: You may be presented for your credentials via the 'Sign in to your account' windows, if you haven't already entered them in this GP session.**
5. In the 'Email Options' window, put your email address in the 'TO...' field, to send the email to, click OK to save changes. Close the User Report Options window.
6. Back in the User Reports window, select this report option and click Insert.
7. Highlight this report option and click the 'Email' button in the menu across the top of this window. You may briefly see a progress window open and close.
8. Verify you receive this email that was just sent. It may take a couple of minutes.
Once you've verified a report such as the above is emailing out successfully, i.e. Modern Auth is enabled and working, then you can look at the template emails such as what you've mentioned.
We usually do this as there are some errors, the 'unknown error' for example, that can show for template issues but also if Modern Auth is not configured correctly, and the troubleshooting for both is different.
Thanks