---
title: Glossary
description: Plain-English definitions of cold email and deliverability terms, from SPF and DKIM to inbox placement, warmup, and complaint rate.
---

## A

### ARF (Abuse Reporting Format)

The standardised format mailbox providers use to report spam complaints back to senders. Warmbly ingests ARF reports where available.

### Alignment

The DMARC requirement that the authenticated domain (from SPF or DKIM) match the visible "From:" domain. Relaxed alignment matches at the organisational level; strict requires exact equality.

## B

### Bounce

A failed delivery. Hard bounces (invalid mailbox, blocked domain) suppress immediately. Soft bounces (mailbox full, temporary outage) are retried with backoff and suppressed if persistent.

### BIMI

Brand Indicators for Message Identification. Lets you publish a verified logo that some clients display next to your messages. Requires DMARC `p=quarantine` or stricter, plus a VMC certificate for full display.

## C

### Cold email

Unsolicited mail to a recipient with whom you have no prior relationship. Distinct from spam by intent: cold mail is targeted, relevant, and gives the recipient a real way to say no.

### Complaint rate

Percentage of delivered messages marked as spam by recipients. Google's bulk-sender threshold is 0.10%. Above 0.30% is in the danger zone.

## D

### DKIM

Cryptographic signature that proves a message was authorised by the domain's key holder and not modified in transit. See [SPF, DKIM and DMARC](/learn/spf-dkim-dmarc/).

### DMARC

The policy that ties SPF and DKIM to the visible From: domain and tells receivers what to do on failure. See [SPF, DKIM and DMARC](/learn/spf-dkim-dmarc/).

## F

### FBL (Feedback Loop)

A direct channel from a mailbox provider that forwards spam-complaint events to the sender so they can suppress the recipient.

## H

### HMAC signature

The cryptographic signature on a webhook delivery that lets you verify the call really came from Warmbly and wasn't tampered with.

## I

### Inbox placement

Share of accepted messages that land in the recipient's primary inbox (not spam, not promotions). The metric that actually matters. See [Inbox placement](/learn/inbox-placement/).

### IP warmup

Gradually increasing send volume from a new IP so receivers learn its behaviour. Required for dedicated IPs; not required for shared IPs you're sending through.

## M

### MTA (Mail Transfer Agent)

The server that sends and routes mail. In cold-email tools, the "sending engine" is effectively the MTA.

## O

### OOO (Out of Office)

Auto-reply indicating the recipient is unavailable. Warmbly classifies OOO replies and suppresses them from sequence engagement metrics.

## P

### Pool (warmup)

A group of warmed mailboxes that send warmup mail to one another. Quality of the pool matters more than size. See [Warmup pools](/learn/warmup-pools/).

### PTR (Reverse DNS)

The DNS record that resolves an IP back to a hostname. Mailbox providers expect outbound mail to come from IPs with matching PTRs.

## Q

### Quarantine

The state of a mailbox temporarily removed from the warmup pool and cold sending. Triggered by reputation warning thresholds.

## R

### Reply detection

Identifying a recipient's reply to a sent message, even when the reply chain has been forwarded or quoted text changes.

## S

### SPF

DNS record listing the servers and services authorised to send mail for a domain. See [SPF, DKIM and DMARC](/learn/spf-dkim-dmarc/).

### Suppression

Excluding a recipient from future sends. Triggered by hard bounce, complaint, unsubscribe, or manual addition.

## T

### TLS

Transport-layer encryption used between mail servers (and between Warmbly and your mailbox provider).

## U

### Unified inbox

A single interface that shows replies across every connected mailbox in a workspace. See the [Unibox guide](/guides/unibox/).
