Email deliverability is the percentage of emails which are delivered successfully to the subscribers' inboxes. It can also be called "inbox placement". It is an important measure of success because emails which are able to reach the targeted inboxes have a great 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 a score of your domain which specifies your level of trustworthiness as a sender. It is indicated based on 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, in unusual formatting or with suspicious URLs trigger ISPs to confirm the content. Rather than that, your content must be high-quality and relevant to your subscriber's wants/needs.
You can see the five causes of poor email deliverability, by returnpath.