Glossary

Email deliverability is the rate at which your messages reach the inbox rather than the spam folder, or nowhere at all. It is shaped by everything in this glossary: authentication records that prove your identity, bounce rates that reveal list decay, and risk signals like spam traps that quietly poison sender reputation. The fastest way to see where one address stands is the free email checker, which returns a verdict in about two seconds.

SPF lists which servers may send mail for a domain, DKIM cryptographically signs each message to prove it was authorized and unaltered, and DMARC tells receivers what to do when a message fails the first two, with reporting back to the domain owner. They work as a set: SPF and DKIM provide the evidence, DMARC sets the policy.

A hard bounce is a permanent failure, usually because the mailbox does not exist, and the address should never be mailed again. A soft bounce is temporary, like a full mailbox or a server that is briefly down, and often resolves on retry. Providers punish repeated hard bounces far more severely, which is why verifying before sending matters.

A catch-all domain accepts mail for every address, valid or not, so a simple existence check cannot tell you whether a mailbox is real. In B2B lists, 20 to 40 percent of addresses typically sit on catch-all domains, which means a large slice of a clean-looking list can hide dead addresses that only reveal themselves as bounces after you send.

A spam trap is an address planted by providers and blocklist operators specifically to catch senders with poor list hygiene. It never opted in, so mailing one signals that you bought, scraped, or failed to clean your list, and a single hit can damage sender reputation or land a domain on a blocklist much faster than ordinary bounces do.

Greylisting is an anti-spam tactic where a receiving server temporarily defers mail from unfamiliar senders, accepting it once they correctly retry. For verification it matters because a greylisting server can make a live mailbox look briefly unreachable, so a naive checker mislabels good addresses while a careful one retries and reads the signal correctly.

They are the rails every check runs on. MX records name the servers responsible for receiving a domain’s mail, and SMTP is the protocol used to ask those servers whether a specific mailbox exists. A verifier looks up the MX records first, then holds an SMTP conversation with the listed server, all without ever sending a message.

Treat both with caution. Disposable addresses self-destruct after minutes, so any send to one eventually bounces. Role addresses like info@ or support@ belong to teams rather than people, and many providers and sending platforms restrict them because nobody opted in personally. Verification flags both so you can decide per campaign.

A blocklist is a list of IPs or domains a mailbox provider treats as untrustworthy, routing their mail to spam or rejecting it outright, and getting delisted takes far longer than getting listed. A whitelist is the opposite: senders trusted enough to skip normal spam scrutiny. Most domains never touch either extreme, but both explain why an otherwise-clean send can suddenly land in spam or sail straight to the inbox.

List hygiene is the ongoing practice of removing invalid, disposable, and risky addresses before they cause damage. Purchased lists carry disproportionate spam-trap and complaint risk since nobody opted in, and zombie emails, ones that technically exist but never open or click, quietly drag down engagement-based reputation even though they rarely bounce. A toxic domain is what you get when enough of these pile up in one segment.

A feedback loop is an arrangement where a mailbox provider notifies a sender whenever a recipient marks their mail as spam, so the sender can suppress that complainer quickly. The junk folder is a softer signal, mail the filter did not trust enough for the inbox but did not reject outright, and landing there repeatedly is an earlier, slower warning than a complaint.

Email validation is the umbrella term for checking that an address is correctly formatted and points to a real, reachable mailbox. Email verification is the fuller process built on top of it, layering syntax, domain, mailbox, and risk checks together. An address that clears every check earns the valid email address verdict.