Email deliverability is how reliably your emails reach the inbox of the people you send them to, rather than being rejected or filtered into a spam or junk folder. It is different from delivery: an email can be accepted by the receiving server, and counted as “delivered” by your platform, yet still land where nobody looks.
How email deliverability works
Every mailbox provider, such as Gmail, Outlook.com, Yahoo and Apple’s iCloud Mail, runs its own filtering. For each incoming message it decides whether to reject it, accept it into the spam folder, or accept it into the inbox (and, in Gmail, which tab).
The decision rests mainly on a few factors:
- Identity. Whether the message passes email authentication (SPF, DKIM and DMARC), proving it comes from the domain it claims.
- Reputation. Your sender reputation, built over time for your domain and the sending IP address from how recipients have treated your past emails.
- Engagement. Whether people open, read, reply, move messages out of spam, delete without reading or report them as spam.
- List quality. How many addresses bounce because they no longer exist, and whether any are spam traps.
- Consistency. Sudden spikes in volume from a sender who usually sends little look suspicious.
- Content. Broken HTML, misleading subject lines or links to poorly regarded domains, though content matters less than the factors above.
Why it matters
An email in the spam folder is close to an email never sent. Worse, problems spread: if your newsletter and your order confirmations go out from the same domain, a careless send of an old list can push receipts and booking confirmations into spam as well. Customers then ring to ask where their confirmation is.
The bar has also risen. Gmail and Yahoo introduced formal requirements for senders in 2024, including authentication, an easy unsubscribe and a low spam complaint rate; Google asks bulk senders to stay below 0.3% and recommends staying under 0.1%. Microsoft followed with similar rules for its consumer mailboxes. These apply to UK businesses as much as anyone; check each provider’s current rules, as this summary reflects them at the time of writing (October 2026).
Common mistakes
- Reading your platform’s “delivered” figure as proof that emails reached the inbox.
- Emailing contacts who have not engaged for years, or a list that was bought or borrowed.
- Sending to everyone at once after months of silence.
- Mixing transactional and marketing email on the same sending domain without thinking about the risk.
- Spending hours removing “spam words” while ignoring authentication and list quality.
How to act on it
Start with the basics you control: authenticate every service that sends as your domain, remove hard bounces automatically, and adopt a sunset policy that stops emailing people who have not engaged for a set period. Register for Google Postmaster Tools to see your Gmail spam complaint rate and whether you meet Gmail’s sender requirements, and use Microsoft’s SNDS if you send from a dedicated IP.
Look at your results by mailbox provider. If Gmail readers click far more than Outlook readers, one provider may be filtering you. Before a large or unusual send, run an inbox placement test. If email is a significant channel for you, its health belongs in your wider digital marketing strategy rather than being left to whoever builds the newsletter.