MailboxValidator

MailboxValidator validates email addresses and cleans lists. Agents run a full single address validation, check whether an address belongs to a free provider, and flag disposable addresses before they enter a list.

Verified 3 actions API key available

MailboxValidator website Platform Documentation Capability data verified 2026-08-19

A signup form posts a new address to the agent

Check Disposable Address answers from the domain alone, before anything slow runs

Check Free Provider marks consumer domains for routing

Validate Email returns the full record, the score, and the verdict

Agent reads the reputation band and separates confirmed failures from unrun checks

Clean addresses flow to the CRM and the ESP with the normalized form attached

The lifecycle owner decides what happens to the addresses whose checks could not run

What This Integration Enables

Most validators hand you a boolean and let you infer the rest. MailboxValidator does something more careful, and it is the reason to choose it: most of its result flags are three valued. A null on is_verified does not mean the mailbox is missing. It means the mail server could not be asked. That distinction sounds academic right up to the afternoon a large provider is slow and a flow that treated falsy as invalid deletes a few thousand real customers. MailboxValidator publishes the server state alongside the verdict so a workflow can tell the difference, and the connector carries the vendor's own score bands through as a reputation field so a flow does not have to rediscover the thresholds.

FlowRunner agents use the three operations for what each is actually good at. The two domain-level checks never open a connection to a mail server, so they are quick enough to run inline while someone is still looking at the form. The full validation is the one that costs a real round trip and returns eighteen signals, a score, a verdict, and the normalized form of the address that deduplication should key on. Errors from this API arrive inside the response body rather than in the status code, including the one that says you are out of credits, and the connector raises each of them as an error so a depleted balance never reads as a clean list. The judgment calls that follow are held by people through human-in-the-loop steps, because the interesting cohort here is never the confirmed failures.

Without FlowRunner

One verdict for two situations A bad mailbox and an unreachable mail server both land in the same reject pile

Validation happens after the fact Bad addresses enter the list at signup and are found later, after they have already bounced

Duplicates survive the dedup The same mailbox enters twice under a plus tag and a dotted variant, and both get mailed

With FlowRunner

The unknowns stay separate Checks that could not run are held as their own cohort, with the server state that caused it

Validation happens at the door Two of the three checks answer from the domain and are fast enough to sit inline on the form

Deduplication on the real mailbox The normalized address is what the agent keys on, so tagged and dotted variants collapse to one

Use Case Scenarios

The form that stops junk without stopping customers

A submission arrives from Typeform or Jotform. Before the agent writes anything, Check Disposable Address runs against the domain, which takes no mail server round trip at all. A throwaway domain is refused on the spot with a message that says so. Check Free Provider then marks consumer domains, which routes the record to self-serve rather than to sales. Only what survives goes to Validate Email for the full record, and only what that clears is written to HubSpot. The visitor experiences one form, and the expensive check runs on a fraction of the traffic.
Deduplication on the mailbox, not the string

An import lands in a staging table in PostgreSQL with addresses collected from three different sources. The agent runs Validate Email across the new rows and keys the dedup on the normalized address the response carries, which has Gmail's dots removed and any plus tagging stripped. Two rows that look like different people to a string comparison collapse into one real mailbox. The surviving row keeps the earliest consent timestamp and the richest field set, and the agent writes a note to a sheet naming every collapse it made so the merge can be reviewed rather than trusted.
A hygiene pass that reports its own blind spots

A quarterly sweep runs Validate Email across a segment. The agent sorts the results three ways rather than two. Confirmed failures with a clear reason go to the suppression list. Clean records with a good reputation band are left alone. Everything else, the records where a check returned null with the server reported down or greylisting the attempt, goes into a third pile with the reason attached. A greylisting server deliberately refuses the first attempt, so the agent schedules that cohort for a later retry and posts the counts to Slack rather than deciding on its own.

Human-in-Loop Highlight

The one place this connector should never act alone is the null verdict. Validate Email returns true, false, or null on most of its flags, and the null on is_verified means the check could not be run, which is a very different thing from a mailbox that does not exist. So the hygiene agent never folds that cohort into the failures. It posts: "Sweep of 24,000 addresses complete. 1,180 confirmed undeliverable and queued for suppression. 640 came back with no verdict: 410 with the server reported down, 230 greylisted on first contact. Retry the greylisted set in six hours, suppress, or hold?" The marketing owner answers. The retry is cheap and the suppression is not reversible from here, because once those addresses are gone from the ESP nothing in this API brings them back. The agent is good at running twenty-four thousand checks. It is not the right thing to decide what an unanswered check means about a customer.

Agent processes routinely

Detects exception requiring judgment

Clear match Continues automatically

Ambiguous Routes to human via preferred channel

Human decides

Agent resumes with decision

Agent Capabilities

3 actions

Validation

Validate Email Runs the full single address check and returns the complete record: syntax, domain, MX, SMTP, catch all, disposable, role, high risk, suppression, and server state, plus the vendor's score and its overall verdict. The connector adds the score's reputation band and carries through the normalized form of the address for deduplication. Used for list hygiene, pre-send sweeps, and any decision that needs more than a domain-level answer.
Check Free Provider Answers from the domain alone whether the address belongs to a consumer mail provider. No mail server is contacted, so it is fast enough to run inline on a form. Used for routing rather than rejection: a consumer domain on a business signup is a lead qualification signal, not an error.
Check Disposable Address Answers from the domain alone whether the address belongs to a throwaway provider. Also contacts no mail server. Used as the first gate on public forms, trial signups, and gated content, where refusing a burner address at the moment of entry is worth more than finding it later.

Frequently Asked Questions

What can FlowRunner do with MailboxValidator?

FlowRunner agents can run Validate Email, Check Free Provider, and Check Disposable Address in MailboxValidator.

Does connecting MailboxValidator to FlowRunner require OAuth?

No. MailboxValidator connects to FlowRunner with an API key, no OAuth flow required.

Can MailboxValidator trigger a FlowRunner workflow automatically?

MailboxValidator doesn't currently expose triggers in FlowRunner. It connects as an action step inside workflows started by another trigger.

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