What is email deliverability, and what are the key factors that affect it?

Deliverability is the ability of an email to reach the recipient's inbox (not spam folder). Key factors:
Sender reputation: Composite score based on IP reputation, domain reputation, engagement rates, spam complaints, and bounce rates.
IP reputation: The sending IP address's history of sending behaviour. New or unwarmed IPs have no reputation — they must be warmed up gradually.
Domain reputation: The From domain's sending history. Major MBPs (Gmail, Yahoo, Outlook.com) now weight domain reputation more heavily than IP reputation.
Authentication: Valid SPF, DKIM, and DMARC alignment are increasingly required by major MBPs. Gmail and Yahoo mandated these from February 2024 for bulk senders.
Content: Spam trigger words, image-to-text ratio, link quality, and HTML coding issues affect spam filter scoring.
Engagement: High open/click rates and low unsubscribe/complaint rates signal to MBPs that recipients want the mail.