Robert Chien's Blog

Below are some commands to capture Mail.app sessions.

In Jaguar the session is recorded in /var/tmp/console.log
In Panther the session is recorded in /Library/Logs/Console/<username>/console.log
In Tiger the session is recorded in /Library/Logs/Console/<username>/console.log

/usr/bin/defaults write com.apple.mail LogActivityOnPort 25
/usr/bin/defaults write com.apple.mail LogActivityOnPort 143
/usr/bin/defaults write com.apple.mail LogActivityOnPort "25,143"

And to disable the logging use this command:

/usr/bin/defaults remove com.apple.mail LogActivityOnPort

Alternately, launch Mail.app in debug mode and send errors to a log file:

/Applications/Mail.app/Contents/MacOS/Mail -LogSocketErrors YES -LogActivityOnHost your.mail.server -LogActivityOnPort 143 &> ~/Desktop/ConnectionLog.txt

Sources http://lists.balius.com/pipermail/mac-users/2005-December/000043.html and
http://developer.apple.com/bugreporter/bugbestpractices.html#Mail

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[UPDATE Nov 21, 2007] More debugging options listed on: http://www.macosxhints.com/article.php?story=2004101603285984