---
title: Email Deliverability Glossary — SendHQ
description: "A plain-English glossary of email and deliverability terms: SPF, DKIM, DMARC, bounce, complaint, SES, transactional email, and more."
canonical: https://sendhq.cc/glossary
last-updated: 2026-08-25
---
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# Email deliverability glossary

The terms that matter when you send email at scale — explained in plain English.

A living reference. New terms added over time.

**SPF (Sender Policy Framework)**

A DNS record that lists which mail servers are allowed to send email on behalf of a domain. Receiving servers check it to reject spoofed senders. A typical record looks like `v=spf1 include:amazonses.com ~all`.

**DKIM (DomainKeys Identified Mail)**

A cryptographic signature added to each email's headers. It lets the receiver verify the message genuinely came from (and wasn't altered since leaving) the sending domain. DKIM uses a set of CNAME records with long tokens.

**DMARC**

Domain-based Message Authentication, Reporting and Conformance. A DNS policy (published at `_dmarc.domain`) that tells receivers what to do when SPF or DKIM fail — none, quarantine, or reject — and where to send aggregate reports.

**Bounce**

An email that could not be delivered. A **hard bounce** means the address doesn't exist (permanent); a **soft bounce** is temporary (full mailbox, server down). Hard-bounced addresses should be suppressed to protect sender reputation.

**Complaint**

When a recipient marks your email as spam or reports it to their provider. High complaint rates damage deliverability, so complaints should be tracked and the affected recipients suppressed.

**Transactional email**

Email triggered by a user's action in your product — password resets, receipts, order confirmations, verification codes. It's expected, time-sensitive, and central to your product's function (as opposed to marketing email).

**Sender reputation**

A score mailbox providers (Gmail, Outlook, etc.) keep for your sending domain/IP, based on delivery history, bounce and complaint rates, and engagement. Good reputation means inbox placement; poor reputation means spam folder or rejection.

**SES (Simple Email Service)**

Amazon's email-sending service. It provides high-deliverability infrastructure for transactional and bulk email, with per-email pricing and detailed delivery, bounce, and complaint feedback.

**Warm-up**

Gradually increasing your sending volume over time so mailbox providers build trust in your new domain or IP. Sending a large volume cold is a common cause of poor initial deliverability.

**Suppression list**

A list of addresses you stop sending to — typically hard bounces and spam complaints. Keeping a suppression list protects your reputation and is required for good deliverability.

**Inbox placement**

The share of your emails that land in the primary inbox (versus the spam or promotions tab). It's the ultimate measure of deliverability health.

**API key**

A secret token your application uses to authenticate to the SendHQ API. Treat it like a password — keep it server-side and never expose it in client-side code.