smtp(s) proxy to monitor outgoing spam

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http://serverfault.com – I am looking for a smtp proxy to install on our gateway which should monitor outgoing smtp traffic to identify the source of recently occuring spam attacks from our network. It would be enough if this could log all outgoing mails, no actual filtering must be done as I'm going to do this manually. Also, is it possible to monitor smtps ports 465 and 587 or is it necessary to completely block these ports to stop spam? (HowTos)