Mailtool does not currently log anything in the Sakai event log. The event log is useful both for site statistics (user activity within site, e.g. as reported by the UFP SiteStats tool), and for research purposes when investigating how users have used Sakai, which in turn is important for informing further development.
Mailtool should post events to the Sakai log (mailtool.*) for each meaningful user action, e.g.
mailtool.new = new message composed/sent
The REF field should include the site id (context) and the message ID number (if there is one) or message subject.
*NOTE: Email Archive already records events using mail.