Writing for the person who signs, not the person who administers
Technical strength is the trap. The best providers write emails full of things the buyer does not evaluate — response times, certifications, stack detail — to a managing director who is thinking about downtime, staff frustration and cost predictability. The draft has to sit in the buyer's language.
Because you read every draft before it sends, that is a correction you can make once. Edit two or three and the rest follow the same register.