Mailbox unavailable. The server response was: 5.7.1 Unable to relay for (email address).

在发送email时得到这个异常，
Mailbox unavailable. The server response was: 5.7.1 Unable to relay for (email address).

解决办法是添加一句代码：

client.DeliveryMethod = SmtpDeliveryMethod.PickupDirectoryFromIis;

新代码如下：

            MailMessage message = new MailMessage(from, to, subject, body);
            SmtpClient client = new SmtpClient();
            client.DeliveryMethod = SmtpDeliveryMethod.PickupDirectoryFromIis;

            message.IsBodyHtml = true;
            client.Send(message);

发信正常了:)