Verify Mailbox Existence with SMTP Handshake
Go beyond domain checks. ValidoAPI performs a real SMTP handshake with the mail server to verify whether the specific mailbox exists — without ever sending an email. The deepest email check available.
Key Benefits
Real SMTP handshake confirms mailbox existence at the server level
No email sent — verification is completely invisible to the recipient
99.5% accuracy for non-catch-all domains — the highest in the industry
Detects invalid mailboxes even on valid, active domains
Reduces hard bounces by eliminating undeliverable addresses before sending
Protects sender domain reputation and ESP sending score
Catch-all domain detection included when SMTP check is inconclusive
Works for both business and free email provider domains
Use Cases
Pre-send Email Validation
Verify every email on your send list before a campaign. Ensure every recipient can actually receive your email at the SMTP server level.
Cold Email Outreach
SDRs sending cold outreach to verified SMTP emails see dramatically lower bounce rates and better deliverability outcomes across sequences.
Transactional Email Flows
Validate customer emails before sending password resets, invoices, and order confirmations. Reduce hard bounces from your transactional ESP.
Real-Time Signup Validation
Verify emails at signup using SMTP to catch deleted or inactive mailboxes that pass DNS and syntax checks but do not actually exist.
ESP Account Protection
Keep your Gmail Workspace, SendGrid, or Postmark sender score healthy by only sending to SMTP-verified addresses.
CRM Qualification
Enrich lead records with SMTP verification status to prioritize outreach toward verified, deliverable email addresses.
Example API Response
{
"email": "john@enterprise.com",
"status": "valid",
"score": 98,
"checks": {
"syntax": true,
"domain": true,
"mx_record": true,
"smtp": {
"connected": true,
"mailbox_exists": true,
"is_catch_all": false,
"smtp_provider": "Microsoft Exchange",
"response_code": 250
},
"disposable": false,
"catch_all": false
},
"response_time_ms": 287
}Frequently Asked Questions
How does SMTP verification work without sending an email?
ValidoAPI connects to the recipient mail server and performs an SMTP dialogue up to the RCPT TO command. This is enough for the server to tell us whether the mailbox exists, without actually queuing or delivering any message.
Is SMTP verification accurate for all domains?
SMTP verification is highly accurate (99.5%+) for non-catch-all domains. For catch-all configured domains, the server accepts all recipients regardless, so we flag these as catch-all and provide a risk score instead.
Will SMTP verification slow down my API responses?
SMTP checks typically add 100–200ms beyond basic DNS checks depending on the remote server. This is the most thorough check available and is worth the extra latency for critical validation scenarios.
Does SMTP verification work for Gmail and Outlook addresses?
Gmail and Microsoft (Outlook/Hotmail) implement anti-abuse measures that prevent direct SMTP verification. For these providers, we use alternative signals and scoring methods combined with syntax and domain checks.
Can SMTP checks get my IPs blacklisted?
ValidoAPI uses a rotating pool of sending IPs and follows best practices for SMTP verification to avoid blacklisting. We manage IP reputation on your behalf so you do not have to worry about it.
Related Services
Start Validating Emails Today
Free plan includes 200 validations/month. No credit card required.
Get Free API Key