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What is SMTP email verification?

Also called: mailbox check, SMTP check, Real-Time SMTP Pinging

SMTP email verification checks whether a mail server will accept a specific address, by opening a connection to the server and asking about the mailbox without sending a message.

By John Britton, VP of Marketing, MedicalProspects · Updated November 4, 2026 · Email & Deliverability

Why it matters for healthcare sales and marketing

It is the fastest way to catch addresses that would hard-bounce, such as people who have left an organization whose mailbox was closed. Running it just before a campaign keeps bounce rates down and protects your sending domain.

How it works

  1. Look up the domain's mail exchange (MX) server.
  2. Open an SMTP connection and identify the sender.
  3. Ask the server whether it will accept mail for the address (the RCPT TO step).
  4. Read the response, then close the connection without sending anything.

Results fall into three groups: valid (the server accepts the address), invalid (it rejects it), and unknown. Unknown results come from catch-all domains that accept every address, from servers that temporarily defer unknown senders (greylisting), and from servers that block verification attempts.

Four steps of an SMTP check: look up the MX server, connect, ask whether the mailbox exists, read the response, with results valid, invalid or unknown.
An SMTP check asks the mail server about a mailbox without sending an email.

Example

Illustrative scenario. A Marketing Ops manager runs an SMTP check on a list of hospital IT directors the night before a campaign. Most addresses come back valid, a few are rejected and removed, and a group on catch-all hospital domains come back unknown. Those go into a smaller test send so the team can watch for bounces before mailing the rest.

Common mistakes

  • Treating "unknown" as valid. Catch-all results can still bounce.
  • Checking too far ahead of the send. Mailboxes close every day; check close to launch.
  • Running aggressive checks from your sending domain. Use a verification service so your sending reputation isn't affected.
  • Stopping at SMTP. A valid mailbox doesn't confirm the person still holds the role.

How MedicalProspects does it

MedicalProspects runs a mailbox check on every email before each delivery, as one layer of its email verification process.

Related terms

  • Email VerificationEmail verification is checking that an email address is correctly formed, belongs to a domain that accepts mail, and points to a mailbox that exists, before you send to it. For healthcare lists it also means confirming the address still belongs to the right person in the right role.
  • Bounce Rate (Hard and Soft Bounces)Bounce rate is the share of sent emails that were returned as undeliverable. Hard bounces are permanent failures, such as an address that doesn't exist; soft bounces are temporary, such as a full mailbox or a server that is briefly unavailable.
  • Deliverability GuaranteeA deliverability guarantee is a data provider's promise that a stated share of the emails it supplies will not hard-bounce, usually backed by replacing or crediting any addresses that do.
  • Data DecayData decay is the gradual loss of accuracy in a contact database as people change jobs, retire, move practices, or their organizations merge and change email domains.

Put it to work

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