What is Deliverability?
Whether your email reaches the inbox (rather than spam, or nowhere) — the product of authentication, reputation, volume, list quality and content.
Delivery is the message being accepted by the receiving server; deliverability is where it lands afterwards. A message can be delivered and still be invisible in the spam folder. Deliverability is not one setting but the sum of authentication (SPF, DKIM, DMARC), sender and domain reputation, sending volume and pacing, the bounce and complaint rates your lists produce, and the content itself (plain text vs. HTML with tracking pixels).
The reliable way to measure it is reply rate on real campaigns plus Google Postmaster Tools; open rate is no longer trustworthy.