What is the primary definition of email deliverability?

The primary definition of email deliverability focuses on the rates of emails that are successfully delivered to recipients' inboxes. This term encompasses various factors that impact whether emails reach their intended targets without being blocked or filtered into spam folders. Ensuring high deliverability means addressing issues like sender reputation, authentication protocols, and engagement metrics, all of which contribute to the likelihood of an email reaching its destination.

The other options address different aspects of email performance but do not define deliverability itself. For instance, the speed at which emails are read relates to engagement but not to whether they were delivered. The volume of emails opened pertains to recipient interaction after delivery but not to the initial delivery rate. Meanwhile, the number of emails marked as spam reflects the recipient's reaction to emails but again does not indicate the overall successful delivery of those emails to inboxes. Thus, focusing on the actual delivery rate provides a clear understanding of what deliverability entails in the context of email marketing.