We are using Siftrock - Siftrock | Email Reply Management for Marketing Automation - to interpret automated email responses. They can detect the "Left Company" case very well, categorize it, and then send that status to Marketo. A Marketo Smart Campaign can then handle the sync to your CRM, the unsubscription or whatever you want to do with these records.
I guess this doesn't really answer your question, as we remove these addresses before they bounce, but I think that's even better.
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