ASP.NET联系人表单问题

我正在为一家小企业设置此表单，因此所有电子邮件都会直接发送到他们的邮件服务器。我输入了正确的信息，邮件从网站上成功发送，但它永远不会到达他们的邮件服务器。他们的邮件服务器确实在联系表格上给出了错误，称5.7.1邮件被内容过滤拒绝为垃圾邮件。如果它没有检测到垃圾邮件，它会发送，但服务器仍然不会收到。

是我的代码出了问题，还是邮件服务器拒绝了它？

c#使用系统；使用System.Net.Mail；

public partial class _Emailer : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack)
        {
            try
            {
                string output = "";
                MailMessage mail = new MailMessage();
                // Replace with your own host address
                string hostAddress = "xxx.xxx.xxx.xxx";
                // Replaces newlines with br
                string message = Request.Form["c_Message"].ToString();
                message = message.Replace(Environment.NewLine, "<br />");
                output = "<p>Name: " + Request.Form["c_Name"].ToString() + ".</p>";
                output += "<p>E-mail: " + Request.Form["c_Email"].ToString() + ".</p>";
                output += "<p>Phone: " + Request.Form["c_Phone"].ToString() + ".</p>";
                output += "<p>Message: " + message + ".</p>";
                mail.From = new MailAddress("xxxxxxx@xxxxxx.org");
                mail.To.Add("xxxxxxx@xxxxxxx.org");
                mail.Subject = "New e-mail.";
                mail.Body = output;
                mail.IsBodyHtml = true;
                SmtpClient smtp = new SmtpClient(hostAddress);
                smtp.EnableSsl = false;
                smtp.Send(mail);
                lblOutcome.Text = "E-mail sent successfully.";
            }
            catch (Exception err)
            {
                lblOutcome.Text = "There was an exception whilst sending the e-mail: " + err.ToString() + ".";
            }
        }  
    }
}

HTML

<asp:label id="lblOutcome" runat="server" />
        <form name="contact" method="post" id="cf">
            <div id="contactform">
                <p><img src="images/required_star.png" alt="Star" /> Required fields for contact form completion</p>
                <ol>
                    <li>
                        <label for="c_Name" class="required-star">Name:</label>
                        <input type="text" id="Text1" name="c_Name" placeholder="John Doe" class="required text" minlength="2" value="<% Response.Write(Request.Form["c_Name"]); %>" />
                    </li>
                    <li>
                        <label for="c_Email" class="required-star">Email:</label>
                        <input type="text" id="Text2" name="c_Email" class="required email text" placeholder="example@domain.com" value="<% Response.Write(Request.Form["c_Email"]); %>" />
                    </li>
                    <li>
                        <label for="c_Phone">Phone:</label>
                        <input type="text" id="Text3" name="c_Phone" class="phoneUS text" placeholder="ex. (555) 555-5555" value="<% Response.Write(Request.Form["c_Company"]); %>" />
                    </li>
                    <li>
                        <label for="c_Message" class="required-star">Message:</label>
                        <textarea id="Textarea1" name="c_Message" rows="6" cols="50" class="required" placeholder="..." minlength="2"><% Response.Write(Request.Form["c_Message"]); %></textarea>
                    </li>
                    <li class="buttons">
                        <input title="Submit" class="buttonBlue" value="Submit" type="submit" />
                        <input title="Clear the form" class="buttonBlue" value="Clear" type="reset" />
                    </li>
                </ol>
            </div>
        </form>