Email Verification: What the Result Codes Actually Mean

Email verification checks whether an address can receive mail without sending anything to it. It is the last step before a campaign and the one most teams run at the wrong time, on the wrong list, and then misread the results.

This is for anyone whose bounce rate is above 2% and who has been told to "clean the list."

How It Works

Three checks in sequence, each cheaper than the one after it.

Syntax; is this a structurally valid address. Catches typos and malformed imports, costs nothing.

Domain and MX; does the domain exist and does it publish mail exchanger records. A domain with no MX cannot receive mail at all, which catches dead companies and misspelled domains.

Mailbox, a conversation with the receiving server asking whether that specific address exists, without delivering a message. This is the expensive step and the one that produces the interesting answers.

The Four Results

Valid

The server confirmed the mailbox exists. Send to these. Expect a small residual bounce rate anyway, because a mailbox can be deactivated between verification and send, which is the entire argument for verifying immediately before a campaign rather than at import.

Invalid

The server said no. Remove them permanently, not just from this campaign. Sending to a confirmed invalid address is the fastest way to damage a sending domain, and these are also where spam traps live.

Catch-all, the one that causes the damage

The domain accepts mail to every address, so the server cannot tell you whether this specific mailbox exists. It says yes to everything.

This is not a validation. It is the absence of one, and treating catch-all as valid is the single most common cause of a surprise bounce rate. The addresses are accepted at the gateway and then silently discarded or bounced later.

Larger enterprises are disproportionately catch-all, so this bucket is not evenly distributed across your list. Segment it separately, send to it at low volume from a domain you can afford to lose, or exclude it from cold campaigns.

Unknown

The server did not answer clearly: greylisting, rate limiting, a timeout. Re-check later rather than guessing; a meaningful share resolve to valid on a second pass a day or two afterwards.

The Thresholds That Matter

Keep hard bounces under 2% and preferably under 1%. Mailbox providers read a high hard-bounce rate as list purchasing, and they are usually right.

That sits alongside the complaint threshold: Google asks senders to stay below 0.1% user-reported spam and treats 0.3% as a hard line. Bounces and complaints are different signals and both are capped tightly, which is why list quality is a deliverability control rather than a marketing preference.

If a campaign bounces above 5%, stop it. Do not finish the send and clean up afterwards, the damage accrues per message.

Verify Once, at the End, Right Before Send

Two sequencing mistakes are near-universal.

Verifying per enrichment pass. If you are running providers in sequence, two of them will frequently return the same address, and verifying after each pass means paying twice for the same check. Merge first, verify the merged set once.

Verifying at import. B2B contact data decays at roughly 2 to 3% a month through job changes and domain migrations, so a list verified in March and sent in September carries six months of rot. Verification is a pre-send step, not an onboarding step.

What Verification Cannot Tell You

Whether the person still works there. A mailbox often stays active after someone leaves: forwarded, monitored by a colleague, or simply not deleted. Verification says the address accepts mail, not that your prospect reads it.

Whether the address is a role account. info@, sales@ and hello@ verify perfectly and perform badly, and they attract complaints at a higher rate. Filter them separately.

Whether it is a spam trap. A recycled trap is a real mailbox that verifies valid. This is the strongest argument against buying lists, no amount of verification detects the problem you bought.

The Working Rules

• Verify the merged set once, immediately before send.
• Remove invalid permanently, not per campaign.
• Treat catch-all as a separate segment, never as valid.
• Re-check unknown after a day rather than guessing.
• Re-verify anything older than a quarter before reusing it.
• Watch bounce rate per sending domain, not just per campaign, so a problem localizes before it spreads.