SocketException but emails are still sent

[jovball]

This is almost certainly not a MailKit problem but I'm hoping you can point us in the right direction for either resolution or troubleshooting.

We are saving mail messages to a pickup folder, loading them and sending them out with MimeMessage/MailKit. This allows us to do additional logging of the messages/recipients and provides (in theory) an easy resend mechanism. We are batching them in groups of no more than 50 per SmtpClient connection but I get the same results when sending a single email per connection.

In the last month, we started having a lot of SocketException error on emails with attachments. These emails are typically 200K - 400K total size with the attachments. Here's the exception message we see.

System.Net.Sockets.SocketException (10060): A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.

We do not receive the 250 OK/QUIT/Bye response from the PostFix mail server and the logical assumption in our code is that this email has not been sent so we retry after a short interval. However, in most cases, it HAS been sent and this results in duplicate emails.

Here is the Smtp Protocol Log output for a single message with two exceptions that was actually sent three times although the lack of response and the SocketExceptions would imply that it was only sent once. I have snipped the MimeContent and redacted emails and other information.

# "Unsuccessful" email on attempt #1 with the socket exception. 

Connected to smtp://csmtp.domain.com:25/
S: 220 gmr01.domain.com ESMTP Postfix
C: EHLO APP-PROD-01
S: 250-gmr01.domain.com
S: 250-PIPELINING
S: 250-SIZE
S: 250-VRFY
S: 250-ETRN
S: 250-STARTTLS
S: 250-ENHANCEDSTATUSCODES
S: 250-8BITMIME
S: 250-DSN
S: 250 SMTPUTF8
C: MAIL FROM:<[email protected]>
C: RCPT TO:<[email protected]>
C: RCPT TO:<[email protected]>
C: RCPT TO:<[email protected]>
S: 250 2.1.0 Ok
S: 250 2.1.5 Ok
S: 250 2.1.5 Ok
S: 250 2.1.5 Ok
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: X-Unsent: 1
C: From: [email protected]
C: Subject: Annual Update Report
C:  40901
C: To: [email protected]
C: Cc: [email protected], [email protected]
C: MIME-Version: 1.0
C: Content-Type: multipart/mixed;
C:  boundary="--_=_NextPart41_8308578f-b902-43d0-ab72-1f9217c8898a"
C: 
C: This is a multi-part message in MIME format.
<snipped>
C: 
C: .


# "Unsuccessful" email on attempt #2 with the socket exception. 


Connected to smtp://csmtp.domain.com:25/
S: 220 gmr02.domain.com ESMTP Postfix
C: EHLO APP-PROD-01
S: 250-gmr02.domain.com
S: 250-PIPELINING
S: 250-SIZE
S: 250-VRFY
S: 250-ETRN
S: 250-STARTTLS
S: 250-ENHANCEDSTATUSCODES
S: 250-8BITMIME
S: 250-DSN
S: 250 SMTPUTF8
C: MAIL FROM:<[email protected]>
C: RCPT TO:<[email protected]>
C: RCPT TO:<[email protected]>
C: RCPT TO:<[email protected]>
S: 250 2.1.0 Ok
S: 250 2.1.5 Ok
S: 250 2.1.5 Ok
S: 250 2.1.5 Ok
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: X-Unsent: 1
C: From: [email protected]
C: Subject:  Annual Update Report
C:  40901
C: To: [email protected]
C: Cc: [email protected], [email protected]
C: MIME-Version: 1.0
C: Content-Type: multipart/mixed;
C:  boundary="--_=_NextPart41_8308578f-b902-43d0-ab72-1f9217c8898a"
C: 
C: This is a multi-part message in MIME format.
<snipped>
C: 
C: .


# Successful email on attempt #3

Connected to smtp://csmtp.domain.com:25/
S: 220 gmr01.domain.com ESMTP Postfix
C: EHLO APP-PROD-01
S: 250-gmr01.domain.com
S: 250-PIPELINING
S: 250-SIZE
S: 250-VRFY
S: 250-ETRN
S: 250-STARTTLS
S: 250-ENHANCEDSTATUSCODES
S: 250-8BITMIME
S: 250-DSN
S: 250 SMTPUTF8
C: MAIL FROM:<[email protected]>
C: RCPT TO:<[email protected]>
C: RCPT TO:<[email protected]>
C: RCPT TO:<[email protected]>
S: 250 2.1.0 Ok
S: 250 2.1.5 Ok
S: 250 2.1.5 Ok
S: 250 2.1.5 Ok
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: X-Unsent: 1
C: From: [email protected]
C: Subject:  Annual Update Report
C:  40901
C: To: [email protected]
C: Cc: [email protected], [email protected]
C: MIME-Version: 1.0
C: Content-Type: multipart/mixed;
C:  boundary="--_=_NextPart41_8308578f-b902-43d0-ab72-1f9217c8898a"
C: 
C: This is a multi-part message in MIME format.
<snipped>
C: 
C: .
S: 250 2.0.0 Ok: queued as E8785C004E6
C: QUIT
S: 221 2.0.0 Bye

Here is the code

 using (var client = new SmtpClient(new ProtocolLogger(smtpLogPath)))
    {
      client.Timeout = (int)TimeSpan.FromMinutes(3).TotalMilliseconds;
        var host = _smtpClientOptions.Server;
        var port = _smtpClientOptions.Port;
        var socketOptions = SecureSocketOptions.None;                
        await client.ConnectAsync(host, port, socketOptions);

        var hasException = true;
// send a batch of messages
        foreach (var m in messages)
        {
            var messageId = Path.GetFileNameWithoutExtension(m.FullName);
            var messageSize = m.ReadableFileSize();
            //assuming failure for logging purposes
            hasException = true;

            try
            {
                var message = MimeMessage.Load(m.FullName);
                var messageSubject = message.Subject;

                if (!client.IsConnected)
                {
                    hasException = false;
                    _logger.LogWarning("Abort sending message {MessageId} and any other staged messages because the client is not connected", messageId);
                    break;
                }

                _logger.LogInformation("Sending message {MessageId} with file size {ReadableFileSize} '{MessageSubject}' to {Recipients}",
                    messageId, messageSize, messageSubject, recipients);
                var smtpServerResponse = await client.SendAsync(message);
                // 2.0.0 Ok: queued as 7347B10001A2" is an example response from the Postfix SMTP server
                _logger.LogInformation("SMTP server response for message {MessageId}: {SmtpServerResponse}", messageId, smtpServerResponse);
                
                //no issues
                hasException = false;
            }
            catch (SmtpCommandException ex) 
            {
                switch (ex.ErrorCode)
                {
                    case SmtpErrorCode.RecipientNotAccepted:
                        _logger.LogError(ex, "SMTP command failure {StatusCode} sending message {MessageFile} with file size {ReadableFileSize} - Recipient not accepted: {Recipient}",
                            ex.StatusCode, m.FullName, messageSize, ex.Mailbox.Address);
                        break;
                    case SmtpErrorCode.SenderNotAccepted:
                        _logger.LogError(ex, "SMTP command failure {StatusCode} sending message {MessageFile} with file size {ReadableFileSize} - Sender not accepted: {Sender}",
                            ex.StatusCode, m.FullName, messageSize, ex.Mailbox.Address);
                        break;
                    case SmtpErrorCode.MessageNotAccepted:
                        _logger.LogError(ex, "SMTP command failure {StatusCode} sending message {MessageFile} with file size {ReadableFileSize} - Message not accepted",
                            ex.StatusCode, m.FullName, messageSize);
                        break;
                    default:
                        _logger.LogError(ex, "SMTP command failure {StatusCode} sending message {MessageFile} with file size {ReadableFileSize}}",
                            ex.StatusCode, m.FullName, messageSize);
                        break;
                }
            } 
            catch (SmtpProtocolException ex) 
            {
                _logger.LogError(ex, "Protocol error failure sending message {MessageFile} with file size {ReadableFileSize}",
                    m.FullName, messageSize);
            }
            catch (Exception ex)
            {                    
                _logger.LogError(ex, "Failure sending message {MessageFile} with file size {ReadableFileSize}",
                    m.FullName, messageSize);
            }
            finally
            {
                if(hasException)
                {
                    resultInfo.AddResultFailure("Failure sending message", $"Message Id: {messageId} - {m.FullName}");
                }
            } 
        }
    }

[jstedfast]

Have you tried checking the postfix server logs?