The Email Glossary
Clear, simple definitions of the terms every developer and marketer needs to know about email delivery, APIs, and authentication.
Blocked Emails
Messages refused by a receiving server for reputation, content, or policy reasons rather than an invalid mailbox.
Bounce Rate
The percentage of sent emails that could not be delivered and were returned by the recipient's mail server.
Bounced Emails
Messages that were rejected by the recipient's mail server and returned to the sender with a status code explaining why.
Click-through Rate (CTR)
The percentage of delivered emails in which a recipient clicked at least one tracked link inside the message.
Dedicated IP vs Shared IP
The tradeoff between sending email from an IP used only by your account and sharing an IP pool with other customers of the same provider.
DKIM (DomainKeys Identified Mail)
A cryptographic signature added to email headers so receiving servers can verify the message was not altered and came from an authorized sender.
DMARC (Domain-based Message Authentication, Reporting & Conformance)
A policy layer on top of SPF and DKIM that tells receivers what to do with unauthenticated mail and provides reports to the sender.
Email API
An HTTP interface that lets applications send, track, and manage email programmatically without speaking SMTP directly.
Email Deliverability
The practice and measurement of ensuring legitimate email actually reaches the recipient's inbox instead of being blocked or junked.
Email Service Provider (ESP)
A platform that sends email on behalf of customers, handling infrastructure, authentication, deliverability, and reporting.
Email Warmup
The practice of gradually increasing sending volume from a new IP or domain to build a positive reputation with mailbox providers.
Global Unsubscribes
A top-level opt-out that removes a recipient from all email a sender could deliver, regardless of category or list.
Inbox Placement
The percentage of accepted emails that land in the primary inbox rather than spam, promotions, or other filtered folders.
Invalid Emails
Addresses that cannot receive mail due to bad syntax, nonexistent mailboxes, or unreachable domains.
List Hygiene
The ongoing practice of removing invalid, inactive, or risky addresses from a sending list to protect deliverability and reputation.
Marketing Email
A commercial message sent to a list of subscribers to promote a product, share news, or nurture leads toward a purchase.
MX Record
A DNS record that tells senders which mail servers accept incoming email for a given domain and in what priority order.
Open Rate
The percentage of delivered emails that were opened by recipients, typically measured via a tracking pixel loaded from the message.
Reported Spam
Messages that recipients marked as spam using the "report spam" button in their email client, generating a complaint signal.
SMTP (Simple Mail Transfer Protocol)
The text-based protocol used to transfer email between mail servers and from clients to servers, defined by RFC 5321.
SPF (Sender Policy Framework)
A DNS record that lists which IP addresses and hosts are allowed to send email on behalf of a domain.
Suppression List
A list of email addresses that must never receive mail from a given sender, used to honor unsubscribes, bounces, and complaints.
Transactional Email
A one-to-one email triggered by a specific user action, such as a password reset, receipt, or shipping confirmation.
Unsubscribe Groups
Named subscription categories that let recipients opt out of specific types of email while staying subscribed to others.
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