Email domains - to subdomain or not?
I have a question regarding email sub-domains/delegated domains and sender reputation:
In the old days of 'Email Marketing' we used to used to use a delegated domain to send emails from our ESP at the time - e.domain.com so emails looked liked email@example.com whereas Joe's real email was firstname.lastname@example.org
This was fine for our mass-communication techniques of the early 2000's - all replies ended up in the marketing mailbox and we had complete control over the sender identity.
Three years ago we moved to a Marketing Automation platform and using DNS authentication, began using the primary domain to send all emails from then on, so Joe's marketing email really went from email@example.com and he would get all the replies (unless we specified otherwise).
We're pretty happy with this setup however when speaking to a consultant about moving to another MA platform they nearly had kittens when we said were sending from the primary domain and recomemnded we go back to using the sub-domains instead for reputation management purposes.
I can't see that it would make any difference as long as all the DNS security was configured correctly, but I am happy to learn something new! (I wasn't in the room when they said this so wasn't able to challenge them directly).
So, is there any reason why they would say this? Has anyone had any experience with good/poor sender reputation using sub-domains vs the primary domain?