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Deliverability

Email verification

Email verification is checking whether an address can actually receive mail before you send to it. A full check tests the syntax, then the domain's mail records, then asks the receiving server about the specific mailbox.

The word covers four checks of wildly different strength, which is why two providers can both sell "verified emails" and deliver very different bounce rates. The weakest is a format check, which only proves the address is shaped like an address. Then a domain check, then an MX record lookup, and only the last step — an SMTP probe against the receiving server — says anything about the mailbox itself.

Even a full probe cannot always give a yes or no. Plenty of servers accept mail for every address at the domain, so the honest answer there is neither valid nor invalid, and a verifier reporting them as valid is guessing on your behalf.

The reason to care is that bounces are the fastest way to damage a sending domain. Mailbox providers read a high bounce rate as evidence you are sending to a list you did not collect yourself, and the reputation damage carries over to every message you send afterwards, including the ones people asked for.

Goes deeper: Email Verification, Explained Properly

Related terms

MX record
An MX record is the DNS entry naming the server that handles mail for a domain. If a domain has no MX record, nothing sent to any address at that domain can be delivered.
SMTP
SMTP is the protocol mail servers use to hand messages to each other. In verification, an SMTP probe opens a conversation with the receiving server and asks whether a specific mailbox exists, without sending anything.
Catch-all email
A catch-all domain accepts mail for every possible address, including ones that do not exist. Because the server says yes to anything, no verifier can confirm whether a specific mailbox there is real.
Bounce rate
Bounce rate is the share of sent messages the receiving server refused. Under 2% is healthy for cold outreach; above 5% and mailbox providers start treating you as a sender who did not collect their own list.
Disposable email address
A disposable address is a temporary mailbox from a service like Mailinator or 10minutemail, used to get past a sign-up form. It usually stops existing within hours.