We're having a vendor send out an email to an external list. We've been asked to provide an unsubscribe link or email address. How do other companies typically handle this when a blast is sent outside of Marketo, and the email addresses will not necessarily be in our system?
You should be able to use a webhook in Marketo to accomplish this. Check out how it works here http://developers.marketo.com/documentation/webhooks/
You can provide to the vendor, your unsubscribe link to the your Marketo instance. Depending on the verbiage in the contract with the vendor, if the person who receives the email is unsubscribing, are they unsubscribing from the vendor or from you. There is no harm in using your unsubscribe link. You will get new names to your data base that are unsubscribed but this ensures that you are complying with their wishes to not receive emails from you.