Issue with Sending Mail in ASP.Net

Probably, your email system requires credentials in order to send.
Here is my generic Email routine:

/// <summary>
/// Send an email from XXX.com
/// </summary>
/// <param name="to">Message to address</param>
/// <param name="body">Text of message to send</param>
/// <param name="subject">Subject line of message</param>
/// <param name="fromAddress">Message from address</param>
/// <param name="fromDisplay">Display neame for "message from address"</param>
/// <param name="credentialUser">User whose credentials are used for message send</param>
/// <param name="credentialPassword">User password used for message send</param>
/// <param name="attachments">Optional attachments for message</param>
public static void Email(string to,
                         string body,
                         string subject,
                         string fromAddress,
                         string fromDisplay,
                         string credentialUser,
                         string credentialPassword,
                         params MailAttachment[] attachments)
    {
    if (to == null)
        {
        to = ConfigurationManager.AppSettings["SMTPDefaultToAddress"];
        }
    string host = ConfigurationManager.AppSettings["SMTPHost"];
    body = UpgradeEmailFormat(body);
    try
        {
        MailMessage mail = new MailMessage();
        mail.Body = body;
        mail.IsBodyHtml = true;
        mail.To.Add(new MailAddress(to));
        mail.From = new MailAddress(fromAddress, fromDisplay, Encoding.UTF8);
        mail.Subject = subject;
        mail.SubjectEncoding = Encoding.UTF8;
        mail.Priority = MailPriority.Normal;
        foreach (MailAttachment ma in attachments)
            {
            mail.Attachments.Add(ma.File);
            }
        SmtpClient smtp = new SmtpClient();
        smtp.Credentials = new System.Net.NetworkCredential(credentialUser, credentialPassword);
        smtp.Host = host;
        smtp.Send(mail);
        }
    catch (Exception ex)
        {
        StringBuilder sb = new StringBuilder(1024);
        sb.Append("\nTo:" + to);
        sb.Append("\nbody:" + body);
        sb.Append("\nsubject:" + subject);
        sb.Append("\nfromAddress:" + fromAddress);
        sb.Append("\nfromDisplay:" + fromDisplay);
        sb.Append("\ncredentialUser:" + credentialUser);
        sb.Append("\ncredentialPasswordto:" + credentialPassword);
        sb.Append("\nHosting:" + host);
        ErrorLog(sb.ToString(), ex.ToString(), ErrorLogCause.EmailSystem);
        }
    }

Ignore the ErrorLog and UpdateEmailFormat methods: you can remove them or write your own.

My web.config file coantains the following in the <appSettings> section:

<add key="SMTPHost" value="LocalHost"/>
<add key="SMTPCredentialUser" value="DoNotReply@XXX.com"/>
<add key="SMTPCredentialPassword" value="XXXX"/>
<add key="SMTPDefaultFromAddress" value="Webmaster@XXX.com"/>
<add key="SMTPDefaultToAddress" value="Webmaster@XXX.com"/>

Try filling in your details, and trying that - I know it works!