Simply getting "Failure to send email"

public Emailer(User user)
{
this.User = user;
this.Client = new SmtpClient("mail.mydomain.com");
this.Client.UseDefaultCredentials = false;

}

public string SendActivationEmail()
{
try
{
this.Client.Credentials = new NetworkCredential("UserRegistration", "password");
MailDefinition md = new MailDefinition();
md.From = "[email protected]";
md.Subject = string.Format("{0}, Welcome to domain.com! Please activate your account.", this.User.UserName);
md.IsBodyHtml = true;
md.BodyFileName = "~/Utilities/Email/EmailConfirmationTemplate.html";
md.EmbeddedObjects.Add(new EmbeddedMailObject("Background", "~/Content/Images/BGPlanet.jpg"));
md.EmbeddedObjects.Add(new EmbeddedMailObject("Logo", "~/Content/Images/logo.png"));

ListDictionary replacements = new ListDictionary();
replacements.Add("{username}", User.UserName);
replacements.Add("{guid}", User.ConfirmationGuid.ToString());
replacements.Add("{benderquote}", Fun.GenerateBenderQuote());

MailMessage message = md.CreateMailMessage("[email protected]", replacements, new System.Web.UI.Control());

Client.Send(message);
return string.Empty;
}
catch (Exception ex)
{
return ex.Message;
}
}

So This code works, right up to Client.Send(message); After about 20 seconds I get. "Failure to send message". No exceptions thrown.

Any Help?