# Deliverability terms, defined

URL: https://www.inboxplease.com/glossary

The records and signals InboxPlease checks, in plain language.

**SPF (Sender Policy Framework)**

A DNS record listing the servers allowed to send email for your domain. Receivers reject or flag mail from servers not on the list.

**DKIM (DomainKeys Identified Mail)**

A cryptographic signature added to each email. It proves the message came from your domain and wasn't altered in transit.

**DMARC (Domain-based Message Authentication)**

A policy that tells receivers what to do when SPF or DKIM fail — none, quarantine or reject — and where to send reports.

**DMARC aggregate report (RUA)**

A daily XML summary from mailbox providers showing which sources sent mail as your domain and whether it passed authentication.

**Alignment (SPF / DKIM alignment)**

Whether the domain that passed SPF or DKIM matches the domain in the visible From address. DMARC requires at least one aligned pass.

**MX record (Mail Exchanger)**

The DNS record that tells the internet which servers receive email for your domain. Missing or broken MX hurts trust.

**Reverse DNS (PTR record)**

Maps a sending IP back to a hostname. Providers distrust IPs without matching reverse DNS.

**Inbox placement**

The share of your emails that land in the primary inbox rather than spam or promotions, measured per mailbox provider.

**Seed test (Placement test)**

Sending an email to a list of monitored mailboxes at each provider to measure where it lands.

**Email warmup**

Gradually increasing sending volume and positive engagement so providers learn to trust a new or recovering domain.

**Blacklist (DNSBL / blocklist)**

A list of domains and IPs flagged for spam or abuse. Providers consult them when deciding where your email goes.

**Sender reputation**

The score providers assign to your domain and IPs based on authentication, complaints, bounces and engagement.

**Spam trap**

An address that never signs up for email. Mailing one signals poor list hygiene and damages reputation.

**Bounce (Hard / soft)**

A message returned as undeliverable. Hard bounces (invalid address) hurt reputation more than soft (temporary) ones.

**MCP (Model Context Protocol)**

An open protocol that lets AI agents call external tools. InboxPlease exposes deliverability tools over MCP.

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