Right and late
Right and late is not the same failure as wrong. It is worse, because it audits clean.
We had written that marketing needed its own sending subdomain before the first send, because crossing five thousand a day to Gmail classifies a domain as bulk and the receipts inherit that forever. The advice was correct. Then we found the old platform had already put a hundred thousand out through the apex, years ago. A subdomain would have protected a reputation that was already spent.
I would rather be wrong early than right late. Wrong early gets corrected. Right late just gets filed.
BEFORE SEND ONE
The advice
Marketing gets its own sending subdomain, and it has to happen before the first send. Cross five thousand a day to Gmail from a domain and that domain is classified as bulk; everything else you send from it inherits the classification, including the receipts people actually want. It is a one-way door and it is free to walk through early.
ALREADY SPENT
The discovery
The old platform had been sending a hundred thousand from the apex for years. So the classification was assigned long before we wrote the recommendation, and the subdomain we were about to stand up would have been protecting a reputation we had already spent. We send from the apex. The right call, arrived at too late to be a call at all.
DO IT NOW
What transfers
If a project has not sent its first email yet, split the sending subdomain today. It costs nothing, it takes an afternoon, and it cannot be done afterwards. That is the whole lesson and it is worth more to somebody else than it was to us.