无法发送带有附件的电子邮件失败（使用System.Net.Mail）

Question

我已经使用下面的代码成功发送了电子邮件：

   Friend Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
      Try
         Dim Smtp_Server As SmtpClient
         Dim e_mail As MailMessage
         Smtp_Server = New SmtpClient()
         Smtp_Server.UseDefaultCredentials = False
         Smtp_Server.Credentials = New Net.NetworkCredential("myuserid@gmail.com", "mypassword")
         Smtp_Server.EnableSsl = True
         Smtp_Server.Port = 587
         Smtp_Server.Host = "smtp.gmail.com"

         e_mail = New MailMessage()
         e_mail.From = New MailAddress(txtFrom.Text)
         e_mail.To.Add(txtTo.Text)
         e_mail.Subject = "Email Sending Subject"
         e_mail.IsBodyHtml = False
         e_mail.Body = txtMessage.Text  ''Take from textbox

         attachment = New System.Net.Mail.Attachment("D:\mypdfdoc.pdf")
         e_mail.Attachments.Add(attachment) 'attachment
         Smtp_Server.Send(e_mail)
         MsgBox("Mail Sent")

      Catch error_t As Exception
         MsgBox(error_t.ToString)
      End Try
   End Sub

但是当我附加文件时：

attachment = New System.Net.Mail.Attachment("D:\mypdfdoc.pdf")
             e_mail.Attachments.Add(attachment) 'attachment

它抛出一个错误： System.Net.Mail.SmtpException：操作已超时。在 System.Net.Mail.SmtpClient.Send（MailMessage消息）

（不带附件，可以通过启用来自Google帐户的安全性较低的应用来正常工作） 哪种解决方案可用。