Email glossary

Plain-English definitions for developers who send email

• BIMIA DNS record (default._bimi.yourdomain) that points to your logo, so supporting mailbox providers can show it next to your emails.
• Bounce (hard and soft)A message from the receiving server saying it couldn't deliver an email. A hard bounce is permanent (the address doesn't exist); a soft bounce is temporary (a full mailbox, or the server is busy).
• Click trackingCounting clicks by sending each link through a redirect that records the click, then forwards to the real page.
• ComplaintA report that a recipient marked your email as spam, passed back to the sender by mailbox providers that offer a feedback loop.
• Dedicated IPAn IP address that sends only one sender's email, instead of an address shared by many senders.
• DeliverabilityWhether your email reaches the inbox, rather than the spam folder or nowhere at all. It depends on authentication, your sending reputation and the content.
• DKIMA cryptographic signature added to every email. The receiving server checks it with a public key published in your DNS, which proves the email came from your domain and wasn't changed on the way.
• DMARCA DNS TXT record at _dmarc.yourdomain that tells receiving servers what to do with email that fails SPF and DKIM alignment (nothing, quarantine it or reject it) and where to send reports.
• Idempotency keyA unique value you send with a request so that retrying it doesn't repeat the action.
• IP warm-upSending a small amount of email from a new IP address or domain at first, and increasing it gradually, so mailbox providers learn to trust it.
• MTA-STSA policy, published in DNS and on an HTTPS URL, that tells sending servers to deliver email to your domain only over an encrypted (TLS) connection with a valid certificate.
• MX recordA DNS record that says which server receives email for a domain or subdomain.
• Open trackingCounting opens with a tiny invisible image in the email: when it loads, the email was (probably) opened.
• Return pathThe hidden address that bounces go back to, separate from the From address people see. SPF is checked against its domain.
• Sender reputationHow much mailbox providers trust mail from your domain and from the IP addresses it's sent from, based on its history: bounces, complaints, spam traps and how recipients engage.
• SPFA DNS TXT record that lists the servers allowed to send email for a domain. Receiving servers check it against the domain in the hidden return path, not the From address you see.
• Suppression listA list of addresses an email service won't send to, usually because they bounced or complained.
• TLS-RPTA DNS TXT record (_smtp._tls.yourdomain) that asks sending servers to report problems they had making encrypted connections to your mail servers.
• Transactional vs marketing emailTransactional email is sent to one person because of something they did: a login code, a receipt, a password reset. Marketing email is sent to many people to promote something: newsletters, offers, campaigns.
• WebhookAn HTTP request a service sends to a URL you choose when something happens, so you don't have to keep asking.