For anyone who is encountering this issue, here is my solution that corrected the authentication issue.
The error above did not properly define what my issue was and upon further investigating from multiple sources was able to isolate to the ServerCertificateValidationCallback. I changed smtp.ServerCertificateValidationCallback = (sender, certificate, certChainType, errors) => true;
to smtp.ServerCertificateValidationCallback = (s, c, h, e) => true;
. See above code for more clarification. With that said, I was able to keep the host name smtp.office365.com and the port 587 with the same credentials. Once this change was made, the email went through no problem.
So, was it an authentication issue with the tenant username/pass? No. The issue at hand was what was being passed into the ServerCerticateValidationCallback. With all of this said, ensure you heed my advice and proceed. IMPORTANT: this does not mean your credentials are completely clean. I wouldn't dismiss the fact it could still be a typo, that you should drop and recreate the tenant with the license or maybe just change the password like others did.
I hope this helps and if you have any other questions related to this, I will be more than happy to answer. Thanks!