All the guys who answered before me had great points! Here's one I didn't see menitoned yet:
----> Send to your to your most engaged recipients first.
The more reliable engagement and open rates from this cohort builds positive reputation, for lack of a better term, with inboxes. It will increase the chances of inboxes receiving the next rounds of emails subsquent to those first successful deliveries.
I use and recommend ActiveCampaign to my clients. They have amazing automations, a nice CRM, and great deliverability. Schedule a call and I'll be happy to walk you through whatever you need.