If an email is scheduled to go out, we do not actually query CRM for the recipients, template, unsubscribes, etc., until the scheduled time (a CRM workflow handles scheduled emails and only submits the request to us when the time occurs). So if CRM is down at that time, when it comes back up, the workflow should submit the request to us immediately and that is when we will begin the process of querying and creating the email messages.
While CRM is down, we queue data for up to 7 days and re-try to sync it once an hour. When CRM comes back up, you will see data being synced within an hour or so.