This weekend, while working on a white paper, I noticed an error in the SDK documentation for the SendEmailRequest.TrackingToken field.  It states:

The tracking token is used to correlate an e-mail with a context. You can generate a tracking token with the GetTrackingTokenEmail message. If this field is unspecified it defaults to null. In this case, the SendEmail message generates a tracking token automatically.

You must supply a value for this field.  The following are all all valid:

SendEmailRequest.TrackingToken = "";
SendEmailRequest.TrackingToken = string.Empty;
SendEmailRequest.TrackingToken = "My Tracking Token";

If SendEmailRequest.TrackingToken is null or undeclared, you will receive an exception.

If SendEmailRequest.TrackingToken is blank, CRM will automatically insert the next available tracking token, such as: CRM:00010029.  If SendEmailRequest.TrackingToken contains a value, that value will be inserted as the tracking token.