How to validate smtp credentials before sending mail?

Question

I need to validate the username and password set in an SmtpClient instance before sending mail. Using this code:

SmtpClient client = new SmtpClient(host);
client.Credentials = new NetworkCredential(username,password);
client.UseDefaultCredentials = false;

// Here I need to verify the credentials(i.e. username and password)
client.Send(mail);

How can I validate whether the user identified by the credentials is allowed to connect and send a mail?

score 4 · Answer 1 · answered Mar 11 '10 at 15:19

4

There is no way.

SmtpClient bascially can not validate the usernamepassword without contacting the serve it connects to.

What you could do is do it outside SmtpClient, by opening a TCP connection to the server... but authentication CAN be complicated depending how the server is configured.

May I ask WHY you need to know that before sending? normal behavior IMHO would be to wrap sending in appropriate error handling to catch exceptions here.

answered Mar 11 '10 at 15:19

TomTom

61,059
10
88
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I needed to know if validating can be done. As it happens in MS outlook that before sending mails, it asks for username and password if it is wrong. I wanted to do the same in my application. I appreciate your help. – Manish Gupta Mar 12 '10 at 04:38
Actually you get something wrong here. MS Outlook does NOT ask you before sending emails. It asks you only in 1 ocurance: it can not authenticate. It tries if you set passord. It may ask if you configured it to do so. But it is perfectly happy to try without password when in doubt. – TomTom Jul 03 '12 at 11:11
I use Healtchecks, it will help to display if the service is correctly configured and the service is available. – Cedric Arnould Jan 14 '20 at 19:28

score 3 · Answer 2 · edited May 23 '17 at 12:09

My production code for username and password validation before sending mail:

public static bool ValidateCredentials(string login, string password, string server, int port, bool enableSsl) {
        SmtpConnectorBase connector;
        if (enableSsl) {
            connector = new SmtpConnectorWithSsl(server, port);
        } else {
            connector = new SmtpConnectorWithoutSsl(server, port);
        }

        if (!connector.CheckResponse(220)) {
            return false;
        }

        connector.SendData($"HELO {Dns.GetHostName()}{SmtpConnectorBase.EOF}");
        if (!connector.CheckResponse(250)) {
            return false;
        }

        connector.SendData($"AUTH LOGIN{SmtpConnectorBase.EOF}");
        if (!connector.CheckResponse(334)) {
            return false;
        }

        connector.SendData(Convert.ToBase64String(Encoding.UTF8.GetBytes($"{login}")) + SmtpConnectorBase.EOF);
        if (!connector.CheckResponse(334)) {
            return false;
        }

        connector.SendData(Convert.ToBase64String(Encoding.UTF8.GetBytes($"{password}")) + SmtpConnectorBase.EOF);
        if (!connector.CheckResponse(235)) {
            return false;
        }

        return true;
    }

More details in the answer to a similar question.

Code on github

Now I'm using lib MimeKit. Work with any types of auth. And works fine in my production. http://www.mimekit.net/ — kazakov.nickolay, Aug 29 '17 at 13:05

score 1 · Answer 3 · answered Mar 11 '10 at 15:24

1

Validating before, sending mail is not possible with the SMTP client class in .net 2.0.

Attempting to validate by hand, by opening a port is as good as writing your own SMTP client, it is complex.

answered Mar 11 '10 at 15:24

panzerschreck

3,582
2
20
29

score 1 · Answer 4 · answered Mar 11 '10 at 15:28

.Net's SmtpClient does not allow you to login without sending a message.

You can check credentials by sending a test message to some extant address that ignores incoming email and checking whether you get an exception.

Note that these test emails will appear in your account's sent folder, if any (eg, Gmail)

keeema · Answer 5 · 2013-09-10T08:57:14.413

0

Try this:

try
{
   smtpClient.Send(new MailMessage("test@test.com", "test@test.com", "test", "test"));
   return string.Empty;
}
catch (SmtpFailedRecipientException)
{
   return string.Empty;
}
catch (Exception ex)
{
   return string.Format("SMTP server connection test failed: {0}", ex.InnerException != null ? ex.InnerException.Message : ex.Message);
}

It works for me in my validation method. But if you only need to check credentials on sending email, then just surround your sending with try...catch.

edited Sep 10 '13 at 08:57

answered Sep 10 '13 at 08:47

keeema

9
2

Test.com is a real domain. I didn't check whether it has an MX record but anybody who just copies your code snippet might cause their mail server to attempt to send real emails to test.com. I recommend that you edit your example with a domain name that truly does not exist (and likely will never exist). – Christoph May 05 '15 at 14:15

score 0 · Answer 6 · answered Apr 29 '10 at 10:32

I agree with the previous answers, that SmtpClient cannot validate without sending.

But maybe your problem can be solved anyway: If username or password are wrong, client.Send(mail); will throw an exception. You can build a while-loop and a try-catch-block around the sending, catch the exception and ask the user for the correct username and password, then try again. If the user clicks cancel on the dialog or sending succeeds without exception, you exit the while-loop.

How to validate smtp credentials before sending mail?

6 Answers6

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