Email Verification & Deliverability Glossary
Glossary

Email Verification & Deliverability Glossary

Plain-English definitions of the email terms that matter: MX records, SPF, DKIM, DMARC, spam traps, catch-all domains, SMTP verification and more.

What Is...?

In-depth definition guides on the core email verification and authentication concepts.

What Is Email Verification?
What email verification is and how it works.
What Is SMTP Verification?
The RCPT TO handshake, response codes, and limitations.
What Is Email Bounce?
Hard vs soft bounces and what they mean for your list.
What Is Disposable Email Address?
Why disposable addresses hurt deliverability.
What Is Catch-All Email?
What catch-all domains are and how to handle them.
What Is MX Record?
The DNS record that routes email to your mail server.
What Is DMARC?
The email authentication policy that stops spoofing.
What Is SPF?
The Sender Policy Framework authorization list.
What Is Spam Trap?
Trap types and how to avoid sending to them.

Is It a Catch-All Domain?

Direct answers on whether each major provider accepts mail to non-existent addresses.

AOL Catch-All? Fastmail Catch-All? Gmail Catch-All? GMX Catch-All? Google Workspace Catch-All? Hotmail Catch-All? iCloud Catch-All? Microsoft 365 Catch-All? Outlook Catch-All? ProtonMail Catch-All? Yahoo Catch-All? Yandex Catch-All? Zoho Catch-All?

Free Email Tools

Check DNS records, catch-all behavior and trap risk without signing up.

MX Lookup
Check a domain's MX records instantly.
Spam Trap Checker
Find trap-like addresses before you send.
DMARC Checker
Read a domain's DMARC policy.
SPF Checker
See which servers are authorized to send.
DKIM Checker
Verify a domain's DKIM signature records.
Catch-All Email Checker
Detect accept-all mail servers.
Disposable Email Checker
Flag temporary and throwaway addresses.

Deliverability Guides

Step-by-step guides to keep your email out of spam and protect sender reputation.

Email Deliverability Guide SPF / DKIM / DMARC Guide List Hygiene

More Terms

Short definitions of the rest of the email vocabulary.

DKIM
DomainKeys Identified Mail - a cryptographic signature attached to outgoing mail that receiving servers verify against a public key in DNS, proving the message genuinely came from the domain and was not altered in transit.
SMTP
Simple Mail Transfer Protocol - the standard protocol used to send and relay email between mail servers on the internet.
RCPT TO
The SMTP command that declares a message recipient. The server's response (250 for accepted, 550 for unknown) is what email verification tools read to determine whether an address exists.
Greylisting
A spam filter that temporarily rejects (451) mail from unknown senders on first contact, then accepts it when the sender retries. It causes verification tools to return "unknown" on the first attempt.
Role Address
An address like info@, sales@, or support@ shared by a team instead of owned by one person. Role addresses carry higher bounce and complaint risk.
Hard Bounce
A permanent delivery failure, usually a 550 response meaning the address does not exist. Hard-bouncing addresses should be removed immediately.
Soft Bounce
A temporary failure such as a full mailbox (452) or busy server (421). A soft bounce does not prove an address is invalid.
DNSBL
A Domain Name System Blacklist - a public list of IPs and domains known for spam. Providers check these lists, and an appearance can block delivery.
Sender Score
A 0-100 reputation score for a sending IP or domain based on complaints, bounces, trap hits, and volume. Higher scores generally mean better deliverability.
Feedback Loop
A mechanism where mailbox providers report spam complaints back to the sender so they can fix the cause before reputation drops.
Return Path
The envelope-from address that receives bounce messages. Keep it monitored and use a verified domain.
IP Warm-Up
Gradually increasing sending volume from a new IP so mailbox providers build a reputation for it before it runs at full capacity.

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Deep-dive guides on how email verification and inbox placement work

Deliverability barometer

Quarterly B2B email deliverability research

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Real lessons from the first cohort

Email verification

How verification works - the 5-stage pipeline, accuracy, and use cases

Inbox Infrastructure Report 2026

272,446-domain census: DMARC gap, provider divide, catch-all rates

Deliverability Benchmarks 2026

10.2M verifications: 12.3% of addresses are dead, and where they hide

Email List Decay Study

826K re-verifications: only 19% of valid addresses survive 90 days

Continue through related topics

Email verification guide Email verification research Email deliverability hub

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