They're almost certainly using a different service for transactional emails than they are for bulk emails. I've used SendGrid and MailChimp together before, an

They're almost certainly using a different service for transactional emails than they are for bulk emails. I've used SendGrid and MailChimp together before, and while I'd avidly recommend either of them to people needing that sort of thing, my biggest pain point was always in the reconciliation between the two.