There could be multiple ways in which you can track and export the email send/engagement data from the Marketo -
1. Email Actvities data - Marketo logs the email delivered, opened, clicked, bounced, etc. actvities in the person activity log, and you can export these engagement data out of Marketo using the bulk extract or query activities endpoint. Since Marketo logs the email activities OOTB for all the people, this solution does not require any custom build/set up in Marketo.
2. Using Program channel and status transitions - if you've proper channel and status setup that you assign people when they engage with the emails, then you can export the "Change Status in Progression" activities using the bulk extract or query activities endpoint.
We are looking for the field in Marketo that tell us a person has been send a specific email so we can do email reporting in Tableau.
OOTB, Marketo does not populate any field with with the email activities/engagements on the person record, rather Marketo logs the activities which you can export as mentioned in the step 1.
Hope this helps. 🙂