Everything worth knowing, nothing that isn't.
How deliverability works
When you send, the receiving mailbox provider checks that the message is authenticated, scores the sending IP and domain reputation, weighs recipient engagement (opens, replies, spam complaints), and then decides inbox, spam, or reject. Getting to the inbox is therefore not about content alone — it is about trust the receiver has built up in your sending identity over time. See /products/email and /answers/what-is-verified-sender.
The three authentication standards
SPF lists which servers may send for your domain; DKIM cryptographically signs each message so tampering and forgery are detectable; and DMARC ties the two together, tells receivers what to do with failures, and reports abuse of your domain. All three aligned is the baseline every mailbox provider now expects before it will trust bulk mail. See /answers/gdpr-for-messaging.
Reputation, warm-up, and list hygiene
Reputation attaches to your sending IP and domain and is built by sending wanted mail consistently. New IPs must be warmed — volume ramped gradually — or providers throttle them. List hygiene keeps it healthy: remove hard bounces, honour unsubscribes, and never send to purchased lists, because spam-trap hits and complaints collapse reputation fast. See /answers/what-is-dnc.
Benefits of getting it right
More email reaching the inbox at the same send volume, fewer bounces and blocks, protection of your domain from spoofing and phishing (a security win as much as a marketing one), and richer analytics on what recipients actually engage with. Strong deliverability makes email a dependable channel in an omnichannel mix rather than a black hole. See /answers/omnichannel-messaging and /products/email.
Limitations and ongoing work
Deliverability is never finished — reputation decays if you send poorly, provider rules change, and one bad campaign can undo months of trust. It also can't rescue unwanted mail: authentication and warm-up get permission-based email delivered, but they won't force content recipients mark as spam into the inbox. Consent and relevance are prerequisites, not optional. See /answers/gdpr-for-messaging.
Why do my emails go to spam even though they're legitimate?
Usually one of four causes: missing or misaligned SPF/DKIM/DMARC so the receiver can't verify you; a cold or poor-reputation sending IP or domain; list problems (bounces, spam-trap hits, low engagement, or purchased lists); or content and sending patterns that look bulk and unwanted. Fix authentication first, then warm the sending identity and clean the list. See /products/email and /answers/what-is-verified-sender.