MailKit: MailKit fails to authenticate against shaw.ca mailserver

My apologies for the vagueness I’m waiting on some more information.

Shaw.ca (ISP in Canada) are running a mail server on mail.shaw.ca and one of my users is attempting to connect to it for their email needs. I THINK it’s running Exchange 2010 but I can’t be certain.

I’ve managed to get an account out of them to test on but to see the problem you actually wouldn’t need one.

I can connect fine

client.Connect(this.ServerAddress, this.ServerPort, SecureSocketOptions.StartTls);

but no matter what I send here

client.Authenticate(this.UserName, this.Password);

no exception is thrown when a logon fails unless I have set the password to blank.

My code then continues having not seen an exception and attempts to send the mail which will return

5.1.0 Authentication required

Using the same details I threw together a test using System.Net.Mail.SmtpClient and that works.