Glossary
Concise definitions for the concepts that appear across this site.
Transactional email
One-to-one email triggered by user action.
Marketing email
One-to-many email sent on the senders schedule.
SMTP
The protocol that delivers email between mail servers.
API vs SMTP
When to use HTTP API versus SMTP relay.
Idempotency keys
A header that prevents duplicate sends on retry.
Event stream
Per-message lifecycle events (delivered, bounced, opened, clicked).
Suppressions
A list of recipients the provider refuses to send to.
Dedicated IP
A sending IP reserved for one customer.
Shared IP
A sending IP used by many customers of a provider.
IP warming
Gradually ramping volume on a new IP to build reputation.
SPF
A DNS record listing which servers may send for your domain.
DKIM
A cryptographic signature attached to outbound mail.
DMARC
A policy that tells mailbox providers what to do when SPF or DKIM fail.
BIMI
A standard for showing your logo next to authenticated mail.
DMARC alignment
The domain match DMARC checks after SPF or DKIM passes.
RUA and RUF reports
The aggregate and forensic report addresses in a DMARC record.
MTA-STS
A policy that tells mail servers to require TLS for inbound delivery.
TLS-RPT
Reports about TLS delivery failures between mail servers.
Hard bounce
A permanent delivery failure.
Soft bounce
A temporary delivery failure.
Complaint rate
The share of recipients who mark email as spam.
Feedback loop
A mailbox-provider feed of spam complaints.
Sender reputation
The trust score mailbox providers infer from sending behavior.
Inbox placement
Whether delivered email lands in the inbox instead of spam.
Seed-list test
A controlled test send to monitored inboxes.
One-click unsubscribe
A standards-based unsubscribe action in the inbox UI.
Return-Path
The envelope sender address used for bounces and SPF alignment.
Spam trap
An address used to detect poor list acquisition or hygiene.
Sandbox mode
A starter mode that limits sending to verified addresses.