// make sure all your constructors call this one.
// this will make the call to base.Send(MailMessage) the default behaviour.
public CustomSmtpMailer()
{
_sendAction = delegate(MailMessage message) { Send(message); };
}
// customizes the SendMail(MailMessage) behaviour for testing purposes.
internal CustomSmtpMailer(SendInternal sendAction)
{
_sendAction = sendAction;
}