Short answer
HubSpot's KB: the Transactional Email Add-On sends transactional emails from a dedicated IP, which only carries transactional email; marketing email on a dedicated IP needs the separate dedicated IP add-on. Keep content to an agreed transaction or updates on one. Buy it through your Customer Success Manager or sales rep, and check content rules with your legal team.
1. Where emails live
The marketing email tool in my demo portal.

2. Not a loophole
Promotions in a transactional email still need marketing consent.