Deliverability is not the same as delivery rate. A campaign can show 99% delivered while the majority of those emails land in the spam folder — counted as delivered by your sender, invisible to the recipient.
Three technical foundations underpin deliverability: SPF (which IPs may send for your domain), DKIM (cryptographic signing of outgoing messages), and DMARC (the policy that tells receivers what to do when SPF or DKIM fails). Without all three configured, major inbox providers progressively throttle or reject your mail.
The non-technical foundation is engagement. Inbox providers — particularly Gmail and Outlook — use recipient signals (opens, replies, deletes without opening, spam reports) to decide future placement. A list that has not been pruned for inactive subscribers in a year will burn your sender reputation; a list that excludes anyone who hasn't engaged in 90 days will preserve it.