问题

I have been trying to implement email functionality in a legacy app using the Legacy Mail API to add OAuth support after getting a token through the Device Code Flow starting with the Microsoft Sample Project.

Along that route I have added SMTP.Send and many other API permissions to find the missing piece. (Including { "User.Read", "User.ReadBasic.All", "SMTP.Send", "offline_access", "Mail.Send" } in fear of missing one)

I have been testing with the MailKit library to build a proof of concept.

So far I have the following Code Snippet that fails after trying to authenticate.

public void SendSmtpMessageAsync(string id, string accessToken)
{
    var message = new MimeMessage();
    message.From.Add(new MailboxAddress("From Name", "From Address@Example.com"));
    message.To.Add(new MailboxAddress("To Name", "To Address@Example.com"));
    message.Subject = "How you doin'?";

    message.Body = new TextPart("plain")
    {
        Text = @"Test Email Content"
    };

    using (var client = new SmtpClient(new ProtocolLogger(Console.OpenStandardOutput())))
    {
        try
        {
            client.Connect("smtp.office365.com", 587, SecureSocketOptions.StartTls);

            var oauth2 = new SaslMechanismOAuth2(id, accessToken);

            var temp = client.AuthenticationMechanisms;
            client.Authenticate(oauth2);

            client.Send(message);
            client.Disconnect(true);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }
    }
}

MailKit Log

I have enabled logging and gotten a log that shows the client connecting than sending the token to authenticate but the authentication failing.

Connected to smtp://smtp.office365.com:587/?starttls=always
S: 220 MW3PR05CA0003.outlook.office365.com Microsoft ESMTP MAIL Service ready at Mon, 25 May 2020 21:31:07 +0000
C: EHLO [192.168.0.7]
S: 250-MW3PR05CA0003.outlook.office365.com Hello [<<My IP>>]
S: 250-SIZE 157286400
S: 250-PIPELINING
S: 250-DSN
S: 250-ENHANCEDSTATUSCODES
S: 250-STARTTLS
S: 250-8BITMIME
S: 250-BINARYMIME
S: 250-CHUNKING
S: 250 SMTPUTF8
C: STARTTLS
S: 220 2.0.0 SMTP server ready
C: EHLO [192.168.0.7]
S: 250-MW3PR05CA0003.outlook.office365.com Hello [<<My IP>>]
S: 250-SIZE 157286400
S: 250-PIPELINING
S: 250-DSN
S: 250-ENHANCEDSTATUSCODES
S: 250-AUTH LOGIN XOAUTH2
S: 250-8BITMIME
S: 250-BINARYMIME
S: 250-CHUNKING
S: 250 SMTPUTF8
C: AUTH XOAUTH2 <<Token omitted but I have confirmed that it is Base64 encoded and 
in the format of base64("user=" + userName + "^Aauth=Bearer " + accessToken + "^A^A")>>
S: 535 5.7.3 Authentication unsuccessful [MW3PR05CA0003.namprd05.prod.outlook.com]
MailKit.Security.AuthenticationException: 535: 5.7.3 Authentication unsuccessful 
[MW3PR05CA0003.namprd05.prod.outlook.com]

Any direction or resources would be appreciated since most existing posts are from pre-2020 when Legacy SMTP support was added. Additionally, if you see any misunderstanding let me know so I can do some additional reading.

回答1:

I had a similar problem, which was cause by my misconfiguration of Microsoft 365. Please see MailKit unsuccessful SMTP OAuth with Microsoft 365 server for details. Hope this helps.

来源：https://stackoverflow.com/questions/62027014/outlook-smtp-oauth-send-authentication-unsuccessful

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outlook