I think you can probably track it down starting from here:
https://github.com/salesagility/SuiteCRM/blob/master/modules/EmailMan/EmailManDelivery.php#L204
but you’ll have to look at those data structures and see how it works.
EDIT: be careful with the database: prospect_lists.domain_name only allows for 255 chars. Once you allow a list, people will say that is small… 