Syntax checks tell you if an address is formatted correctly. SMTP verification tells you if the mailbox actually exists. BounceZero does both — plus catch-all detection, greylisting retry, and ML scoring for providers that block probes.
BounceZero's SMTP verification process goes through four distinct phases to confirm real mailbox existence.
Resolve the domain's MX records to identify which mail servers accept inbound email for that domain.
Open a TCP connection to the highest-priority MX server. Issue EHLO and MAIL FROM to initiate an SMTP session.
Issue RCPT TO with the target address. A 250 OK response confirms the mailbox exists; 5xx confirms it doesn't.
Run 3 additional RCPT TO probes with random addresses. If all return 250, the domain is a catch-all — classified as Risky.
Not all mail servers cooperate with SMTP probes. BounceZero has engineered solutions for every edge case.
Some servers temporarily reject (450) first-time SMTP connections. BounceZero retries after the greylisting period with exponential backoff, ensuring accurate results without false undeliverables.
Gmail and Microsoft block RCPT TO probes, returning false positives. We detect these providers and switch to domain intelligence, Gravatar lookups, breach correlation, and ML scoring for accurate results.
Domains configured to accept all RCPT TO commands would appear as Deliverable to a naive checker. Our 3-probe algorithm uses random addresses to fingerprint this behavior and flag it as Risky/Catch-All.
200+ provider profiles capture unique server behaviors, rate limits, banner formats, and response codes. The right verification strategy is applied automatically based on the detected provider fingerprint.
Not all verification methods are equal. Here's how they compare.
| Method | What It Checks | Accuracy | Catch-All |
|---|---|---|---|
| Syntax / Regex only | Format correctness | Low | ✗ No |
| DNS / MX lookup | Domain has mail servers | Medium | ✗ No |
| Basic SMTP RCPT TO | Mailbox acceptance | Medium-High | ✗ No |
| BounceZero 5-Stage | All of the above + ML + reputation + catch-all | 99.5% | ✓ Yes |
An SMTP email check verifies whether a specific address can receive messages by connecting to the mail server and issuing a RCPT TO command — the same protocol used in real email delivery — without sending an actual message. A 250 OK response confirms the mailbox exists; a 5xx response confirms it doesn't.
Gmail and Microsoft block RCPT TO probes and always return 250, making direct SMTP verification unreliable for these providers. BounceZero detects them automatically and switches to domain intelligence, Gravatar presence lookups, breach data correlation, and ML scoring to deliver high-confidence results without SMTP.
A catch-all domain is configured to accept any inbound email at any address, even if the specific mailbox doesn't exist. This makes simple SMTP verification useless — every address looks deliverable. BounceZero detects catch-all behavior by running 3 RCPT TO probes with random addresses and classifying these results as Risky/Catch-All.
No. SMTP verification uses the SMTP protocol but terminates the connection after the RCPT TO command — before any message content is transmitted. No email is ever sent to the address being verified. The target mailbox owner has no way of knowing a verification was performed.
Try the free single verifier or sign up for bulk verification and API access.