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dsniff is general purpose password sniffer, it handles *lots* of different protocols, but it also handles tcp-style expressions for limiting analyzed traffic - so I can limit it to work on pop3 only.
The command is useful for monitoring the use of the boxes and their connection IP.
Result file "sniff" is readable with GUI program "wireshark" or through CLI with the command:
tcpdump -f "sniff" -XX
Of course, you can adjust "Maildir" to your config...