ARC (Authenticated Received Chain)
A protocol that preserves original authentication results across intermediaries like mailing lists and forwarders.
Glossary
Email deliverability glossary.
Plain-language definitions for email deliverability and infrastructure terms: authentication, reputation, compliance, and the API concepts behind them.
Read the deliverability blog or the API docs for how these terms apply in practice.
Authentication
BIMI (Brand Indicators for Message Identification)
A standard that displays a verified brand logo next to authenticated emails in supporting inboxes.
DKIM (DomainKeys Identified Mail)
A cryptographic signature added to outgoing mail that lets receivers verify the message wasn't altered in transit.
DMARC (Domain-based Message Authentication, Reporting & Conformance)
A DNS policy that tells receivers what to do with mail claiming to be from a domain that fails SPF and DKIM alignment.
DMARC alignment
The requirement that the domain in SPF or DKIM matches (exactly or by organizational domain) the visible From address.
MTA-STS
A policy mechanism that requires SMTP delivery to a domain to use TLS with a valid certificate, preventing downgrade attacks.
Return-Path (envelope sender / bounce address)
The address bounce notifications are sent to, which SPF actually authenticates instead of the visible From address.
SPF (Sender Policy Framework)
A DNS TXT record listing which mail servers are allowed to send email for a domain.
Reputation & deliverability
Blocklist (DNSBL)
A published list of IP addresses or domains flagged as sources of spam, checked by receiving mail servers in real time.
Bounce classification
Sorting a non-delivery response into a category (hard, soft, block, etc.) that determines whether and how to retry.
Complaint rate
The percentage of delivered messages a recipient marks as spam, one of the most heavily weighted deliverability signals.
Engagement-based filtering
Mailbox providers using recipient behavior (opens, clicks, deletes without reading) as a primary signal for inbox versus spam placement.
Feedback loop (FBL)
A subscription with a mailbox provider that forwards spam-complaint notifications back to the sender in near real time.
Hard bounce
A permanent delivery failure, typically because the recipient address doesn't exist, that should never be retried.
Inbox placement rate
The share of delivered messages that land in the primary inbox rather than a spam or promotions folder.
IP/domain warm-up
A gradual, engagement-weighted volume ramp used to establish reputation for a new sending IP or domain.
List hygiene
Ongoing practices that keep a mailing list free of invalid, unengaged, or risky addresses before they damage reputation.
Seed testing
Sending to a panel of monitored test addresses across mailbox providers to check where a message actually lands before or after a real send.
Sender reputation
A mailbox provider's ongoing assessment of a sending IP and domain, built from bounce, complaint, and engagement history.
Soft bounce
A temporary delivery failure, such as a full mailbox or a transient server issue, that is safe to retry.
Spam trap
An email address that was never legitimately opted in, used by mailbox providers and blocklists to identify poor list-acquisition practices.
Infrastructure
Dedicated IP
A sending IP address used exclusively by one sender, whose reputation reflects only that sender's own sending history.
Lane isolation
Routing transactional and marketing (or per-tenant) traffic through separate sending identities and IP pools so one traffic class can't damage another's reputation.
MTA (Mail Transfer Agent)
The software that routes and delivers email between mail servers over SMTP.
MX record
A DNS record that specifies which mail servers accept email for a domain, and in what priority order.
PTR record (reverse DNS)
A DNS record that maps a sending IP address back to a hostname, checked by many receivers as a basic legitimacy signal.
Shared IP pool
A set of sending IP addresses used by multiple senders, whose collective reputation each tenant relies on.
SMTP (Simple Mail Transfer Protocol)
The internet-standard protocol used to transmit email between mail servers.
Throttling (rate limiting)
Deliberately capping the rate of outbound connections or messages to a receiving domain to stay within its tolerance and avoid triggering blocks.
Compliance & consent
CAN-SPAM Act
The US federal law setting baseline requirements for commercial email: accurate headers, a working unsubscribe, and a physical postal address.
Consent evidence
A retained, timestamped record of how and when a recipient agreed to receive marketing email, kept for compliance review.
Double opt-in
Requiring a recipient to confirm their email address via a follow-up link before they're added to a sending list.
GDPR consent (email context)
The EU/UK GDPR requirement that marketing email generally needs a freely given, specific, informed, opt-in legal basis before sending.
List-Unsubscribe header
An email header that lets recipients unsubscribe with one click from within their mail client, without opening the message.
Suppression list
A record of addresses that must never receive mail again, populated from unsubscribes, hard bounces, and complaints.
API & delivery
Bulk email (marketing email)
Email sent to many recipients at once for promotional or informational purposes, not triggered by an individual user action.
Delivery event
A discrete, timestamped record of something that happened to a specific email after it was sent: accepted, delivered, bounced, opened, and so on.
Idempotency key
A client-supplied identifier that lets an API safely retry a request after a timeout without risking a duplicate send.
Postmaster tools
Free dashboards mailbox providers (notably Google and Microsoft) offer senders to monitor domain reputation and delivery signals directly.
Sending domain
The domain used in the From address of outgoing mail, which is where authentication and reputation are anchored.
Transactional email
Email triggered by a specific user action or system event, such as a password reset, receipt, or shipping notification.
Webhook
An HTTP callback the sending platform makes to a customer's endpoint to report a delivery event as it happens.