Unable to Send Email MailBox UnAvailable 5.7.1

 Hi Guys,
I am kind of new to Asp.Net. I am working on a website which has a "contact us" page. I am taking the sender's email address and his question from the page and I am trying to send that as an email. I am getting an error which says:
"MailBox UnAvailable Server Response was: 5.7.1 Message rejected as spam by content filtering".
I am using the following code to send email:
                MailMessage mail = new MailMessage();
                mail.To.Add(new MailAddress(toAddr));
                mail.From = new MailAddress(fromAddr);
                mail.Subject = subject;
                mail.ReplyTo = new MailAddress(fromAddr);
            ...