Email deliverability expresses what share of sent messages reaches the recipient's main inbox, rather than the spam or promotions folder. The gap between emails sent and emails delivered tends to be surprisingly large, and most companies never track it at all, because the statistics show the number of pieces sent. Deliverability is determined mainly by the reputation of the sending domain and server, which is built up over time: how many people open the messages, how many mark them as spam, how many addresses are invalid, and whether you are sending to people who asked for it. The technical foundation is correctly configured sender authentication; the content foundation is a clean contact list and relevant content. You can recognise deteriorating deliverability from a drop in open rate without any change in content, most often after a bulk import of old addresses.
See also: SPF, DKIM and DMARC, Contact list hygiene, Unsubscribe and complaint rate.
