Deliverability
Whether your email reaches the inbox rather than spam — or nothing at all.
Deliverability is the share of your sent email that lands where people will see it. It is not the same as the delivery rate: an email accepted by the receiving server and filed into spam counts as delivered and is still a failure.
It is decided mostly before the email is written. Authentication records, the reputation of the sending domain, how the list was collected and how recipients react all weigh more than the wording of a subject line.
The parts you can act on directly: authenticate the domain with SPF, DKIM and DMARC; keep the list clean; make unsubscribing easy; and send to people who asked for it.