Email deliverability measures the ability to deliver email messages in recipients’ inboxes for email marketing campaigns and to avoid messages being lost, blocked or driven to the spam folder. Mailchimp defines it as "A measure of success at which an email marketer gets an email campaign into subscribers’ inboxes."
By extension, email deliverability also encompasses the secondary folder issue. Marketers have to be in the inbox but also to be in the good folders and to avoid junk mail or if possible the Google’s promotion tab.
Email deliverability is an issue due to address quality and spam filtering. Issues associated with address quality are easily managed through good addresse verification and list hygiene practices. Deliverability issues due to spam filtering and blocking are more complex to manage and may affect legitimate emails sent to optin subscribers.
There are two main indicators or metrics for measuring deliverability: "classic" deliverability rate and inbox placement rate. Each is a based on a different measurement methodology and inbox placement is often seen as more accurate.
Email marketers use many techniques and processes to ensure deliverability.