Can You Auto-Unsubscribe Retirees and Employees No Longer at Company?

YES! This is super helpful and we do that with Siftrock (now owned by Drift). It connects to our email inboxes and automatically categorizes email responses as OOO, Left Company, Human Reply, etc., and allows us to add them to separate Marketo lists. Smart Campaigns then block emails going to those who have left the firm.

Well, Marketo cannot monitor your inbox for the auto-responder emails - one way is to build a service or use a third party app (if you search for track email replies in the community you can get some suggestions on 3rd party apps) that could initiate a workflow of updating the Unsubscribed field of the person from whom you receive these emails to update the Unsubscribe field to True using Marketo API (you can run this service on a daily/weekly basis). The other way is have set up a custom activity and have the service create custom activities upon email reception - custom activity could then trigger a campaign setup to update the Unsubscribed field to TRUE.

Marketo campaigns cannot directly filter people based off on the auto responder email reception to an email address.