Per offline SCRUM discussion: add e2e NTLM/Kerberos testing for SmtpClient authentication.
Example:
https://github.com/dotnet/corefx/blob/master/src/System.Net.Security/tests/FunctionalTests/NegotiateStreamKerberosTest.cs

/cc: @karelz @Priya91