Can you give a little more clarity on how they accidentally unsubscribed? I'm wondering if there might be something else preventing them from receiving the newsletter such as hitting their email limit for the day/week or possibly bouncing.
Agree with Chris Saporito.
Have you checked whether Marketo attempted to send the email? Try searching to see whether this person is a member of a newsletter program, or looking at whether there's anything in their activity log that suggests an attempt to send was made. It's likely that:
- They did receive the email but it fell into their spam box/they missed it
- The system attempted to send the email, but it bounced
- The system did not attempt to send the email as the lead was at their weekly/daily email limit, or was blocked by another system field (e.g. Marketing Suspend)
- The system did not attempt to send the email because the lead did not meet the send criteria.