Hello Jon,
I'm a member of Marketo's Privacy Team. We handle email deliverability and compliance.
The unsubscribe status has legal weight to it in most countries. In the US, the CAN-SPAM act requires that you stop sending promotional material to email addresses that have unsubscribed, and there are possible legal penalties to ignoring this. Even outside of the legal penalties, chances are high that people would mark the messages as spam and complain to our abuse team, which could result in impaired deliverability and potential abuse investigations.
There are two cases in US law where you are legally allowed to email unsubscribed addresses: relationship messages, and transactional messages. You can use these to potentially win back a few folks.
A relationship message is a message that talks about changes to an existing business relationship, and only discusses these terms. An example would be a notice of technical downtime, or a notice that a company is changing their terms of service.
A transactional message is a message that someone requests and receives in response to that request. An example would be a receipt - someone makes a purchase, and expects a receipt in email. Another example is a whitepaper download or event registration.
Neither of these message types can contain promotional offers or marketing content if you send them to unsubscribed leads.
Even though you can't email marketeing content to unsubscribed people, you can email these people when appropriate with relationship and transactional messages. You can craft your campaigns or use dynamic content to note when a lead is unsubscribed, and include a link to resubscribe in these messages.
In other words, if someone is unsusbscribed but decides to register for a webinar, your registration message could also include a short note like, "We notice you're unsubscribed from our regular emails. We've added new subscription options to help you receive just the messages you want. Click here to learn more!"
A strategy like this can allow you to invite engaged leads to resubscribe when you're in contact for other reasons, while still respecting their usubscribed status. This should also keep you on the right side of both the law, and the Marketo abuse team.