The afterSubmitContacts callback is for you. This callback is invoked after the user clicks the Next button on the Choose Your Contacts page and before the widget overlay is dismissed. The callback receives the array of selected contacts, the name of the source of the contacts, and the owner contact.
Only the contacts that the user selected are passed to the function. All these contacts will have at least one email address. If the user abandons the import process, this callback will not be invoked.