We work hard to keep delivery rates and domain reputation as high as possible. If we use broadcast for transactional emails it hurts the reputation, and if we need to register a different domain to separate them, we need to build that reputation from scratch.

If we can link the email address to a specific SMPT, then each email would be channeled accordingly. newslettter@ for broadcasts, info@ for transactional emails... all within one domain with different SMPTs.