C＃：发送邮件失败

Question

我已经尝试了多种方法，例如：

• 使用不同的版本，端口，打开或关闭SSL，...
• 其他邮件服务提供商
• 在我的Gmail帐户设置中设置“允许不太安全的应用”

题：

• 如何解决此错误并成功发送附件？
• 是什么导致此错误？
• 是因为端口可能被阻塞了吗？

错误：

Exception thrown: 'System.IO.IOException' in System.dll
Exception thrown: 'System.Net.Mail.SmtpException' in System.dll
System.Net.Mail.SmtpException: Failure sending mail. ---> System.IO.IOException: Unable to read data from the transport connection: net_io_connectionclosed.
   at System.Net.Mail.SmtpReplyReaderFactory.ProcessRead(Byte[] buffer, Int32 offset, Int32 read, Boolean readLine)
   at System.Net.Mail.SmtpReplyReaderFactory.ReadLines(SmtpReplyReader caller, Boolean oneLine)
   at System.Net.Mail.SmtpReplyReaderFactory.ReadLine(SmtpReplyReader caller)
   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)
   --- End of inner exception stack trace ---
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   at TreeView.TreeView.<Email>d__28.MoveNext() in C:\Users\Alexander\Documents\Visual Studio 2015\Projects\TreeView\TreeView\TreeView\TreeView.cs:line 365

码：

public void Email()
{
    try
    {
        SmtpClient client = new SmtpClient()
        {
            Host = @"smtp.gmail.com",
            Port = 465,
            Credentials = new NetworkCredential(@"mailid@gmail.com", @"Password"),
            UseDefaultCredentials = false,
            DeliveryMethod = SmtpDeliveryMethod.Network,
            EnableSsl = true
        };

        MailMessage message = new MailMessage()
        {
            From = new MailAddress(@"mailid@gmail.com"),
            Subject = @"Test",
            Body = @"Test",
            Priority = MailPriority.Normal
        };

        message.To.Add(@"mailid@example.com");
        Attachment file = new Attachment(@"C:\Users\Alexander\Documents\File.txt");
        message.Attachments.Add(file);

        client.Send(message);
    }
    catch { }
}

Answer 1

对于gmail我有

SmtpClient client = new SmtpClient();
client.Host = "smtp.gmail.com";

client.Port = 587;

client.EnableSsl = true;
client.Credentials = new NetworkCredential("mygmailusername@gmail.com", "mygmailpassword");
                ////email.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network ;

PS您可能需要启用此功能：

https://support.google.com/accounts/answer/6010255?hl=zh_CN

附加：

下载此工具：

https://www.microsoft.com/zh-cn/download/details.aspx?id=24009

并查看是否可以查询该端口/地址

我得到以下结果：

=============================================

 *Starting portqry.exe -n smtp.gmail.com -e 587 -p TCP ...
Querying target system called:
 smtp.gmail.com
Attempting to resolve name to IP address...
Name resolved to 74.125.29.109
querying...
TCP port 587 (unknown service): LISTENING
portqry.exe -n smtp.gmail.com -e 587 -p TCP exits with return code 0x00000000.*

Answer 2

设置UseDefaultCredentials将始终将Credentials设置为null 。

因此，必须先设置UseDefaultCredentials = true/false然后再设置Credentials 。

参考： https : //stackoverflow.com/a/27896975/5075227