TPE
You first need to upgrade to v1.17.3, install the latest build of the OAuth2 plugin (if you're using it), and retest. If you continue to have issues check the email in question's SMTP information and attempt to save changes with SMTP enabled to see if it authenticates successfully. If it does authenticate successfully, then you will need to contact your mail administrator to see why your email is being marked as spam.
Cheers.