How verification works

Every address goes through the same steps. The cheap, certain checks come first, so a mail server is only asked when it has to be.

1. Basic checks

For every address:

• Syntax — is it a well-formed email address?
• Disposable domain — is it a throwaway inbox provider?
• Spam traps — does it look like a known or likely trap? These are never contacted.
• Administrative addresses — abuse@, postmaster@ and similar are never contacted.
• Mail server (MX) lookup — does the domain receive mail at all? A domain with no mail server, or one that says it takes no mail, makes every address on it No Mail Server.
• Parked domains — domains parked at a domain seller aren’t checked further.

2. Answers we already have

• If a mailbox told us “does not exist” in the last 45 days, we reuse that answer instead of asking again.
• A definite answer for the same address is reused for 48 hours, so the same list uploaded twice in a row gets the same results.

3. Asking the mail server — without sending an email

For everything left, EmailShield connects to the recipient’s mail server and starts the conversation a real email would — hello, this is who the mail is from, this is who it’s for — and stops there. The server answers whether it would accept mail for that mailbox. No message is ever sent, and the person never sees anything.

This is done from dedicated verification infrastructure, at a steady, polite pace for each receiving provider, so servers keep giving honest answers.

4. Catch-all test

Once per domain, a realistic made-up address is tested as well. If the server accepts that too, the domain accepts everything and its answers can’t confirm a mailbox. More about catch-alls →

5. Microsoft 365

For domains hosted on Microsoft 365, EmailShield asks Microsoft’s directory. A positive answer becomes Valid (MS365). A negative isn’t trusted — the directory leaves out aliases and shared mailboxes — so it’s followed up with a mail-server check instead of being called invalid.

6. Retries

“Try again later” answers are retried after a pause; timeouts are retried by a different route with longer waits. A definite “this mailbox doesn’t exist” is final straight away. After a few attempts without an answer, the address is Unknown and refunded.

What EmailShield doesn’t do

• It never sends an email to the addresses on your list.
• It never guesses. An address without a real answer is Unknown, not valid.
• It doesn’t promise deliverability. A valid mailbox can still filter your email to spam; that depends on your domains, your inboxes and what you write. See Maildeck’s deliverability guides.