使用企业邮箱发送运行时生成的PDF附件邮件失败求助

问题描述 #

使用企业邮箱发送带运行时生成PDF附件的邮件时，出现以下错误：

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Authentication Required. Learn more at
或
Failure sending mail

相关代码如下：

protected void BtnSendEmail_Click(object sender, EventArgs e)
{
    Send_Email("moin.softwin@gmail.com","This is a mail with an attchment!","hello There, Have a good day. I have sent u an email with an attchment!");
}

protected string Bind_Invoice()
{
    string op = null;
    try
    {
        con = new SqlConnection(CommonClass.Get_DB_Path);
        con.Open();
        cmd = new SqlCommand("BIND_PURCHASE_INVOICE", con);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.CommandTimeout = 10000;
        cmd.Parameters.AddWithValue("@USER", Session["username"].ToString());
        SqlParameter RuturnValue = new SqlParameter("@RETVAL", SqlDbType.VarChar, 18000);
        RuturnValue.Direction = ParameterDirection.Output;
        cmd.Parameters.Add(RuturnValue);
        cmd.ExecuteNonQuery();
        op = cmd.Parameters["@RETVAL"].Value.ToString();
        cmd.Dispose();
        con.Close();

    }
    catch (Exception ex)
    {
        if (ex.Message != null)
        {
            op = ex.Message;
            con.Close();
        }
    }
    return op;
}

protected void Send_Email(string ToAddress, string MailSubject, string MailBody)
{
    try
    {
        MailMessage message = new MailMessage();
        message.To.Add(ToAddress);// Email-ID of Receiver  
        message.Subject = MailSubject;// Subject of Email  
        message.From = new System.Net.Mail.MailAddress("MYUSERNAME");// Email-ID of Sender  
        message.IsBodyHtml = true;

        string PDFdata = Bind_Invoice();
        MemoryStream file = new MemoryStream(PDFGenerate(PDFdata, Server.MapPath("img/bg.jpg")).ToArray());

        file.Seek(0, SeekOrigin.Begin);
        Attachment data = new Attachment(file, "RunTime_Attachment.pdf", "application/pdf");
        ContentDisposition disposition = data.ContentDisposition;
        disposition.CreationDate = System.DateTime.Now;
        disposition.ModificationDate = System.DateTime.Now;
        disposition.DispositionType = DispositionTypeNames.Attachment;
        message.Attachments.Add(data);//Attach the file  

        message.Body = MailBody;
        SmtpClient SmtpMail = new SmtpClient();
        SmtpMail.Host = "MY HOST ADDRESS";//name or IP-Address of Host used for SMTP transactions  
        SmtpMail.Port = 465;//Port for sending the mail  
        SmtpMail.Credentials = new System.Net.NetworkCredential("MYUSERNAME", "MYPASS");//username/password of network, if apply  
        SmtpMail.DeliveryMethod = SmtpDeliveryMethod.Network;
        SmtpMail.EnableSsl = true;
        SmtpMail.UseDefaultCredentials = false;
        SmtpMail.ServicePoint.MaxIdleTime = 0;
        SmtpMail.ServicePoint.SetTcpKeepAlive(true, 2000, 2000);
        message.BodyEncoding = Encoding.Default;
        message.Priority = MailPriority.High;
        SmtpMail.Send(message); //Smtpclient to send the mail message  
        Response.Write("Email has been sent !");
    }
    catch (Exception ex)
    { 
        Response.Write("Failed !"); 
    } 
}

private MemoryStream PDFGenerate(string message, string ImagePath)
{
    MemoryStream output = new MemoryStream();
    Document pdfDoc = new Document(PageSize.A4, 25, 10, 25, 10);
    PdfWriter pdfWriter = PdfWriter.GetInstance(pdfDoc, output);
    pdfDoc.Open();
    Paragraph Text = new Paragraph(message);
    pdfDoc.Add(Text);
    byte[] file;
    file = System.IO.File.ReadAllBytes(ImagePath);
    iTextSharp.text.Image jpg = iTextSharp.text.Image.GetInstance(file);
    jpg.ScaleToFit(550F, 200F);
    pdfDoc.Add(jpg);
    pdfWriter.CloseStream = false;
    //pdfDoc.Close();
    output.Position = 0;
    return output;
}

解决方案 #

1. 校正SMTP基础配置 #

• 确认服务器地址：替换MY HOST ADDRESS为企业邮箱官方提供的SMTP域名（如腾讯企业邮为smtp.exmail.qq.com，阿里企业邮为smtp.mxhichina.com），禁止使用错误的IP或非官方域名。
• 匹配端口与加密规则：端口465对应SSL加密，但部分企业邮箱要求使用587端口（STARTTLS协议），需查阅企业邮箱文档确认。若切换到587端口，保持EnableSsl = true即可。

2. 修复认证信息问题 #

• 使用完整邮箱用户名：企业邮箱SMTP认证的用户名必须是完整邮箱地址（如user@company.com），不能仅填前缀，替换代码中所有MYUSERNAME为完整邮箱。
• 验证密码有效性：若企业开启了二次验证，需使用应用专用密码而非原登录密码；未开启则确保MYPASS是当前有效的邮箱登录密码。
• 调整认证参数顺序：确保UseDefaultCredentials = false在设置Credentials之前执行，避免.NET默认覆盖自定义认证信息，修改代码片段：

  SmtpMail.UseDefaultCredentials = false;
  SmtpMail.Credentials = new System.Net.NetworkCredential("完整邮箱地址", "密码/应用专用密码");
  

3. 优化代码细节 #

• 捕获完整异常信息：修改catch块输出详细错误，定位问题根源：

  catch (Exception ex)
  { 
      Response.Write($"发送失败：{ex.Message}，详情：{ex.InnerException?.Message}"); 
  }
  

• 释放PDF资源：取消PDFGenerate方法中pdfDoc.Close();的注释，确保PDF文档资源正确释放，避免内存流损坏导致发送失败。
• 统一编码格式：将message.BodyEncoding = Encoding.Default改为Encoding.UTF8，避免中文乱码或编码不兼容问题。

4. 检查企业邮箱权限 #

• 联系管理员确认该账号是否被允许使用SMTP服务，以及服务器IP是否在邮箱的发送白名单内。
• 登录企业邮箱后台，确认SMTP服务是否已手动开启（部分云邮箱默认关闭该功能）。

内容的提问来源于stack exchange，提问作者USER0349579345