ASP.NET WebForms C#邮件发送失败问题排查求助

问题背景 #

ASP.NET WebForms站点发送邮件时，能正常发送到部分地址，但无法送达目标地址，抛出连接中止相关异常。

邮件发送代码 #

protected void btnSend_Click(object sender, EventArgs e)
{
    string Name = txtName.Text;
    string EmailAddress = txtEmail.Text;
    string phone = "Number not supplied";

    if (txtPhone.Text != "")
    {
        phone = txtPhone.Text;
    }

    string SendAddress = @"emailsender@address.com";
    string Subject = "Message from Top Tree Fellas Website";
    string Message = Regex.Replace(txtMsg.Text, @"\r\n?|\n", "<br />");

    if (String.IsNullOrEmpty(Name) || String.IsNullOrEmpty(EmailAddress) || String.IsNullOrEmpty(Message))
    {
        lblError.Text = "Please ensure all required fields are filled in.";
    }
    else
    {
        try
        {
            MailMessage msg = new MailMessage();                                
            msg.To.Add(new MailAddress(@"email@address.co.uk"));
            msg.CC.Add(new MailAddress(@"email@address2.com"));
            msg.Subject = Subject;
            msg.From = new MailAddress(SendAddress);

            msg.IsBodyHtml = true;

            msg.Body = "<p><strong>Contact Name:</strong> " + Name + "<br />";
            msg.Body += "<strong>Email Address:</strong> " + EmailAddress + "<br />";
            msg.Body += "<strong>Phone:</strong> " + phone + "<br />";
            msg.Body += "<strong>Message:</strong><br />" + Message + "</p>";

            msg.ReplyToList.Add(new MailAddress(EmailAddress));

            SmtpClient smtpClnt = new SmtpClient("smtp.ionos.co.uk");
            //smtpClnt.UseDefaultCredentials = false;
            smtpClnt.Credentials = new NetworkCredential(@"emailsender@address.com", "password");
            smtpClnt.EnableSsl = true;

            smtpClnt.Send(msg);

            lblError.ForeColor = System.Drawing.Color.Green;
            txtName.Text = "";
            txtEmail.Text = "";
            txtMsg.Text = "";
            txtPhone.Text = "";

            Response.Write("<script>alert('Message sent');</script>");
        }
        catch (Exception er)
        {
            string er1 = er.Message + "<br />" + er.InnerException;
            lblError.Text = er1;
            Response.Write("<script>alert('Something went wrong. Please try again later or call us on: 07719 306404');</script>");
        }
    }
}

报错信息 #

Failure sending mail

内部异常 #

System.IO.IOException: Unable to read data from the transport connection: An established connection was aborted by the software in your host machine.
System.Net.Sockets.SocketException: An established connection was aborted by the software in your host machine
at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
--- End of inner exception stack trace ---
at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
at System.Net.FixedSizeReader.ReadPacket(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
at System.Net.Security._SslStream.StartFrameHeader(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
at System.Net.Security._SslStream.StartReading(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
at System.Net.Security._SslStream.ProcessRead(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
at System.Net.TlsStream.Read(Byte[] buffer, Int32 offset, Int32 size)
at System.Net.PooledStream.Read(Byte[] buffer, Int32 offset, Int32 size)
at System.Net.Mail.SmtpPooledStream.Dispose(Boolean disposing)
at System.IO.Stream.Close()
at System.IO.Stream.Dispose()
at System.Net.ConnectionPool.Destroy(PooledStream pooledStream)
at System.Net.ConnectionPool.PutConnection(PooledStream pooledStream, Object owningObject, Int32 creationTimeout, Boolean canReuse)
at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint)
at System.Net.Mail.SmtpTransport.GetConnection(ServicePoint servicePoint)
at System.Net.Mail.SmtpClient.GetConnection()
at System.Net.Mail.SmtpClient.Send(MailMessage message)

排查思路与解决建议 #

• 指定SMTP端口：Ionos的SMTP服务启用SSL时常用465或587端口，当前代码未设置端口，默认25端口可能被服务商禁用。添加smtpClnt.Port = 465;或587测试。
• 修复凭据配置：取消注释smtpClnt.UseDefaultCredentials = false;，确保先禁用默认凭据再设置自定义SMTP凭据，避免冲突。
• 检查目标邮箱反垃圾策略：目标地址email@address.co.uk可能将邮件标记为垃圾，查看该邮箱的垃圾邮件文件夹，或把发件地址加入白名单。
• 验证服务器网络限制：站点所在服务器的防火墙/安全组可能阻止了SMTP出站连接，联系主机商确认是否允许访问Ionos的SMTP端口。
• 强制TLS版本：旧版.NET Framework默认SSL/TLS版本可能不兼容Ionos服务，在创建SmtpClient前添加代码：ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;。
• 简化收件人测试：暂时移除CC地址，只发送到目标地址，排除多收件人引发的问题。
• 验证SMTP凭据有效性：用邮件客户端（如Outlook）配置相同的SMTP服务器、账号和密码，测试能否发送，确认凭据是否正确。

内容的提问来源于stack exchange，提问作者M_Griffiths