Siftrock (a Marketo plug-in for reply handling and database health) can help do tracking and measurement on replies in some cases (I say some because there are certain implementations that Siftrock can't support).
The basic idea: Siftrock becomes a virtual inbox to collect all your campaign replies. It uses machine learning to find the real human responses (and classified the OOOs, auto-replies, left companies, soft bounces, etc). Then it syncs back to Marketo by adding people who replied onto a static list and/or setting fields that you determine.
Referenced on this thread as well: Email Reply Analysis Strategies
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