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I have a program on Windows that sends email via a remote SMTP server, and it is failing with an ambiguous "cannot connect" error message with no log or other information as to why it is failing. I have tested the SMTP server and confirmed it is working with the same credentials.

The program uses plaintext login to the server, as it doesn't support SSL/TLS, so the SMTP connection should be transmitted in the clear. Is there an easy way to use a packet sniffer to monitor the attempted connection, to see exactly what is being sent (in an effort to discover why the connection is failing)?

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Install that sucker, fire it up, start a capture, and Bob's yer uncle.

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Yes there is. Install a packet capture program on the client (Wireshark, Microsoft Network Monitor, etc.) and start a capture, then start your program and invoke the email function, then look at the capture. You can filter the capture by protocol (SMTP) or by source/destination ip address to filter all of the "noise" out of the capture to allow you to focus on only the conversation that you're interested in.

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