Email distribution solution

We are sending out email weekly blasts to approx. 20,000 customers every week via platforms like mailchimp. Whenever we add new email addresses we have a risk of 5-8 initial complaints and a risk of being blocked by mailchimp etc.. We are running huge new personalized campaigns to other new industry types and need a technical explanation why and what is the best way to do this. We don't want a freelancer to build an infrastructure, but are willing to pay for orienting is to the right place/technique, to give us the necessary answers to our questions. Two freelancers were referred to us by another freelancr and they will be invited to this project.