I experienced the same issue. The System Admin email address i specified in the Jenkins config did not exist in office 365. Thank you for posting the solution and saving me hours of troubleshooting.
Hi Steven Haehn,
I am getting the same error even after adding java arguement -Dmail.smtp.starttls.enable=true and admin email "mye...@company.com".
Could you please help me. attaching the screenshot.
Thanks,
Rakesh