Email deliverability is defined as the percentage of emails which are successfully delivered to subscribers' inboxes. It can also be called "inbox placement". It is an important measure of success because emails which successfully reach the targeted inboxes have a greater chance of being opened.
Email deliverability is influenced by three core factors:
1. Identification: Gatekeepers of a subscriber's inbox or ISP determine whether you are using a set of standard protocols (Domain-Based Message Authentication, Reporting and Conformance (DMARC), Domain Keys Identified Mail (DKIM) and Sender Policy Framework (SPF)).
2. Reputation: Reputation is your domain's score which specifies your level of trustworthiness as a sender. It is indicated on the basis of your blacklist listings, bounce rates, complaint rates, and email volume.
3. Content: The quality of your content has a significant effect on whether or not your email will be delivered to the recipient’s inbox. Emails with spammy subject lines, unusual formatting or suspicious URLs trigger ISPs to confirm the content. Your content must be high-quality and relevant to your subscriber's wants/needs.
You can see the five causes of poor email deliverability here, by returnpath.