A plain-English email glossary covering the authentication, deliverability, infrastructure and security terms you will meet across the email ecosystem. Each definition links to our in-depth guide where there is more to learn. Bookmark it as a quick reference, and explore the full pillars from our hubs on authentication, deliverability, SMTP services and security.
Email Health Platform terms
Email Health
The overall condition of your email setup — how well your domain authenticates, routes and secures mail, and how good its sending reputation is. ToolTrusted measures it across four dimensions: authentication, infrastructure, security and reputation. Run a free Email Health Check to see yours.
Email Health Check
ToolTrusted’s free diagnostic that runs several DNS checks on any domain in one pass (SPF, DKIM, DMARC, MX, MTA-STS, TLS-RPT, BIMI and a live blacklist lookup) and returns an Email Health Score with the exact fixes ranked. It is the entry point to the platform. Run the Email Health Check.
Email Health Score
A single grade out of 100 (A–F) summarising a domain’s email health, with a sub-score for each of the four dimensions — authentication, infrastructure, security and reputation. The score is the public output of ToolTrusted’s Email Health Engine; deliverability is reported as honest, non-scored intelligence rather than a fabricated number.
Email Health Report & Workspace
The Email Health Report is a saved, shareable result for a domain (the same dimension breakdown as the live check, kept at a permalink). The free Email Health Workspace is where you save reports, track your score’s history over time, and turn on monitoring so you are alerted when something changes.
Email authentication terms
SPF (Sender Policy Framework)
A DNS record listing the servers authorised to send email for your domain. Receivers check it against the connecting server’s IP. See our SPF guide.
DKIM (DomainKeys Identified Mail)
A cryptographic signature added to each message so receivers can verify it came from your domain and was not altered. See our DKIM guide.
DMARC
A policy that ties SPF and DKIM to your visible From address and tells receivers what to do with mail that fails (none, quarantine or reject). See our DMARC guide.
Alignment
The DMARC requirement that the SPF- or DKIM-authenticated domain match the domain in the visible From header. Without alignment, DMARC fails even when SPF or DKIM passes.
BIMI
Brand Indicators for Message Identification — a standard that displays your verified logo beside authenticated mail, requiring DMARC at enforcement. See our BIMI guide.
MTA-STS
A standard that requires inbound mail to your domain to use validated TLS encryption, preventing downgrade attacks. See our MTA-STS guide.
TLS-RPT & ARC
TLS-RPT reports on the success or failure of encrypted delivery; ARC preserves authentication results across forwarders and mailing lists that would otherwise break DMARC.
DNSSEC
DNS Security Extensions — cryptographically signs your DNS records so resolvers can verify they are authentic and unaltered, protecting the SPF, DKIM, DMARC and MX records email relies on from spoofing. It is also a prerequisite for DANE. See our DNSSEC for email guide.
Infrastructure terms
SMTP
Simple Mail Transfer Protocol — the universal standard for sending email between servers. See our SMTP services hub.
Email API
An interface for sending email programmatically over HTTP, with templating, webhooks and analytics. See our email APIs hub.
MX record
The DNS record that names the mail servers which receive email for your domain, in priority order. See our MX record guide.
Reverse DNS (PTR)
A DNS record mapping a sending IP back to a hostname; mailbox providers require valid, matching reverse DNS from senders. See our reverse DNS guide.
Dedicated vs shared IP
A dedicated IP is used by one sender alone (full control, needs warm-up and volume); a shared IP is pooled across senders (inherits reputation, no warm-up). See our sender reputation guide.
Deliverability terms
Inbox placement
The share of delivered mail that lands in the inbox rather than spam — the metric that actually matters, distinct from delivery rate. See our inbox placement guide.
Sender reputation
The trust score mailbox providers assign to your sending domain and IP, built from complaints, engagement, bounces and authentication. See our sender reputation guide.
Warm-up
Gradually increasing volume from a new IP or domain so providers build a reputation from real engagement before full-volume sending. See our warm-up guide.
Spam trap
An address used to catch senders with poor list hygiene; pristine traps were never opted in, recycled traps are abandoned real addresses. Hitting them can get you blacklisted.
Blacklist (DNSBL)
A DNS-published list of IPs or domains known for spam, queried by receivers in real time. See our email blacklist guide.
Hard vs soft bounce
A hard bounce is a permanent failure (invalid address) — suppress immediately; a soft bounce is temporary (full mailbox, throttle) — retry. See our bounce guide.
Complaint rate & feedback loop (FBL)
The complaint rate is the share of recipients marking you as spam (keep it under 0.1%); a feedback loop reports those complaints so you can suppress the senders. See our bounce & complaint guide.
Throttling & greylisting
Throttling is a provider temporarily limiting your sending (a 4xx deferral); greylisting defers the first attempt from an unknown sender to filter spam. Both call for measured retries, not aggression.
Security & list terms
Email spoofing
Forging the sender address so mail appears to come from a trusted source; stopped on your domain with DMARC at enforcement. See our spoofing guide.
Phishing
Fraudulent email that tricks recipients into revealing data, paying fake invoices or installing malware — usually built on spoofing. See our phishing guide.
Email encryption (TLS, S/MIME)
TLS encrypts the connection between mail servers; S/MIME and PGP encrypt the message contents end to end. See our encryption guide.
List hygiene & verification
Keeping your list free of invalid, risky and inactive addresses, often using an email verification tool, to protect deliverability. See our verification hub.
Suppression list
The set of addresses you must never send to again — hard bounces, complaints and unsubscribes — checked before every send to protect your reputation.
Opt-in & one-click unsubscribe
Opt-in means recipients actively consented to your mail (confirmed opt-in adds a verification step); one-click unsubscribe is the one-step opt-out now required of bulk senders by Gmail and Yahoo.