Is there a way to re-send failed transactional emails using SparkPost email extension?
I have two outside ideas that 'might' be helpful:


Is it possible Activities have been made for the emails that should have gone out? Emails from the Contact record will almost certainly have created an activity, and it's a commonly set option for emails sent via Scheduled reminders. If an activity has been made it will probably contain the email.
Do you have email logging turned on? Presumably not or you wouldn't be asking the question! But perhaps worth checking...