I can confirm that from my optusnet account in MS Outlook 2019:
-I can email several gmail users, but not others (no error, but email doesn't reach them)
-I can't email to any Yahoo user (again no error, but emails don't reach them).
I have run "repair" account, scanpst, and repair Office, but none help.
I also tried your Outlook settings, but no luck - either an error message saying something went wrong and to retry... or no error but email doesn't reach them.