SMTP AUTH on MailShark must be used with TLS => v1.0.
Without a secure connection SMTP AUTH credentials (your username and password) in delivered in clear text so can easily be hacked.
For SMTP AUTH to work on MailShark you need:
- TLS => v1.0
- SSL <= v3.0 is not supported, contains too many vulnerabilities to make it useful in live internet services
For MS Exchange servers, you would typically need to enable "Basic Authentication over TLS" and click OK.