Data validation glossary
Every term here appears in a real lookup result or in the data behind one. Definitions are written so that a result and its definition can never disagree.
Catch-all domain
A mail domain that accepts every address, so a positive SMTP answer proves nothing about a specific mailbox.
SMTP probe
Opening a real SMTP conversation with the receiving mail server to ask whether a mailbox exists, without sending a message.
MX record
The DNS record that names which servers accept mail for a domain, and in what order to try them.
Disposable email address
A throwaway mailbox from a temporary-mail provider, designed to receive one signup confirmation and then be abandoned.
Role address
An address that belongs to a function rather than a person, such as info@, support@ or sales@.
Free mail provider
A consumer mailbox host such as Gmail or Outlook, as opposed to a company-owned domain.
SPF
A DNS record listing which servers are allowed to send mail for a domain.
DKIM
A cryptographic signature added to outgoing mail so the receiver can verify the message really came from the domain and was not altered.
DMARC
A published policy telling receiving servers what to do with mail that fails SPF and DKIM, and where to send reports.
Greylisting
A spam defence that temporarily rejects mail from unfamiliar senders, expecting legitimate servers to retry.
Hard bounce
A permanent delivery failure: the address does not exist and never will.
Soft bounce
A temporary delivery failure, such as a full mailbox or a busy server, where a retry may succeed.
Spam trap
An address kept alive specifically to catch senders who mail people who never opted in.
Sender reputation
The running score mailbox providers keep on a sending domain and IP, which decides whether mail reaches the inbox.
E.164
The international standard phone number format: a plus sign, country calling code, and national number, with no spaces or punctuation.
NPA-NXX
The first six digits of a North American number: the area code plus the exchange, which together identify the block a number was assigned from.
Line type
Whether a number is a mobile, a landline, or a VOIP line.
Mobile number portability
The right to keep a phone number when switching carriers, which breaks the link between a number's prefix and its current network.
HLR lookup
A live query against the mobile network's Home Location Register to see whether a handset is currently reachable and which network serves it.
Do Not Call registry
A government list of numbers whose owners have opted out of telemarketing calls.
TCPA
The US Telephone Consumer Protection Act, which restricts automated calls and texts and sets statutory damages per violation.
Robocall
An automated call delivering a prerecorded message, and the largest single source of consumer phone complaints.
ASN
An Autonomous System Number: the identifier for a network that routes its own block of IP addresses on the internet.
Datacenter IP
An address belonging to a hosting or cloud provider rather than to a consumer internet connection.
Proxy and VPN detection
Identifying that a connection is being relayed, so the visible address is not where the user actually is.
Tor exit node
The last relay in the Tor network, whose IP address is what a destination site sees.
DNSBL
A DNS-based blocklist: a list of IP addresses or domains published so that servers can query it during a connection.
Public Suffix List
A maintained list of the suffixes under which the public can register domains, used to find the real registrable domain.
Domain age
How long ago a domain was first registered, one of the plainest signals separating established sites from disposable ones.
Tranco rank
A research-grade popularity ranking of domains, built to resist the day-to-day manipulation that affects raw traffic lists.
OFAC SDN list
The US Treasury list of Specially Designated Nationals, the people and entities US persons are prohibited from dealing with.
KYB
Know Your Business: verifying that a company counterparty is real, identifiable, and not sanctioned before doing business with it.
FCRA
The US Fair Credit Reporting Act, which governs data used to decide eligibility for credit, insurance, employment, or housing.