SMTP Email Verification API: What It Checks and What It Costs

An SMTP email verification API asks the recipient's mail server whether it would accept a mailbox, then returns valid, invalid or risky without sending a message. On SimpleVerifier it is included with the $39 a month Pro plan: 10,000 calls a month, then $1.50 per 1,000. You can test the same checks with 100 free checks, no card.

This post covers what the SMTP step does, how to handle the awkward outcomes in code, and what the API costs next to other vendors.

What the SMTP step does

The API looks up the domain's mail servers, connects, and walks the first steps of sending a message: it names a sender and then asks whether the recipient is accepted. A clear "no such user" answer means invalid. A clear accept means the mailbox likely exists. It then closes the connection before any message is sent. Read SMTP verification false positives and SMTP deferral versus bounce for the cases where the answer is not clear.

Handling results in your code

• valid: accept the address.
• invalid: reject it and ask the user to correct it.
• risky: catch-all domains, timeouts and greylisting land here. Accept with a note, or queue for later, but do not treat it as valid.

Add a timeout and a fallback so a slow server never blocks a signup. See the real-time email verification API guide and the API quickstart.

What to look for

1. A real SMTP check, with a reason field, not syntax only.
2. Clear handling of catch-all, disposable and role addresses.
3. Predictable pricing: included calls and the overage rate. Our API cost per month post shows how to count calls.
4. Documentation, errors and SDKs. Start at the API docs.

How SimpleVerifier fits

Pro is $39 a month with 10,000 calls included, then $1.50 per 1,000 on monthly billing; Business includes 100,000 and Agency 300,000. Pro also covers CSV verification, the first 100,000 checks a month at full speed, then slower, never cut off, so one subscription covers forms and lists. Compare options on the email verification API and validation API for developers pages, and try a check on the verify page.

Price comparison from public pricing pages

Figures come from public pricing pages as recorded in our comparison data. Confirm before buying.

Option	What it sells	Checked
Abstract API	$39 a month for 15,000 requests	12 Oct 2026
Mailboxlayer	$74.99 a month for 50,000 requests	12 Oct 2026
Kickbox	Pack: 10,000 for $70	3 Nov 2026
MillionVerifier	Pack: 10,000 for $39	2 Nov 2026
SimpleVerifier Pro	$39 a month, 10,000 API calls included, then $1.50 per 1,000	Our own plan

When another vendor is cheaper

If your API need is a steady 15,000 calls a month, Abstract API's $39 tier covers it with the quota included, where Pro would bill overage past 10,000. If you verify through the API occasionally in bursts, a prepaid pack that never expires, such as MillionVerifier's 10,000 for $39, may cost less than a subscription. Pro is the better value when you also run CSV lists, since the same $39 covers both. See the Abstract API comparison.

Bottom line

Choose an SMTP API on its handling of risky results and on the cost at your real call volume. Test with the free checks before you integrate.