try modifying this segment of code:
SmtpServer.Credentials = new NetworkCredential("myEmailAddress", "myPass");
SmtpServer.EnableSsl = true;
SmtpServer.UseDefaultCredentials = false;
to this:
SmtpServer.UseDefaultCredentials = false;
SmtpServer.Credentials = new NetworkCredential("myEmailAddress", "myPass");
SmtpServer.EnableSsl = true;
In other words, try disabling UseDefaultCredentials before setting the credentials to use.
The order that the properties are set may make a difference.
I'm pretty sure I've seen a similar question and reordering things worked. I could be misremembering, though.