Email Verifier

Verify whether email addresses are deliverable by connecting to the recipient's mail server and checking if it accepts the address — without actually sending any mail.

How It Works

1. 1. MX Lookup — Resolves the domain's mail exchange server
2. SMTP Handshake — Connects to the MX server on port 25
3. RCPT TO Check — Asks the server if it would accept mail for the address
4. Catch-All Detection — Tests a random address to detect catch-all domains

Dependencies

CODEBLOCK0

Usage

Single or multiple emails

From stdin

From CSV (e.g., a lead list)

Options

• - --helo DOMAIN — HELO domain for SMTP greeting (default: verify.local)
• INLINECODE1 — Connection timeout (default: 10)

Output

JSON array to stdout. Each result contains:

CODEBLOCK4

Deliverability values

Rate Limiting

The script includes built-in rate limiting to protect your IP reputation:

CODEBLOCK5

Options

• - --delay SECONDS — Pause between each check (default: 1.0)
• INLINECODE7 — Max checks to one domain before pausing (default: 20)
• INLINECODE8 — How long to pause after hitting the per-domain limit (default: 30)

Why rate limiting matters

SMTP verification connects directly to mail servers. Without rate limiting:

• - Your IP gets blacklisted — Mail servers (especially Gmail, Microsoft) flag IPs that make many rapid RCPT TO requests. Once flagged, your IP may be blocked for hours or permanently.
• Port 25 gets blocked — ISPs monitor outbound port 25 traffic. Unusual volume can trigger automatic blocks.
• Greylisting increases — Servers that see rapid-fire checks start returning temporary failures, making your results less accurate.
• It looks like spam reconnaissance — Because that's exactly what spammers do. Legitimate use requires responsible pacing.

Guidelines for agents

Rule of thumb: Stay under 50 unique domain checks per day from a residential IP. For repeated checks to the same domain (pattern guessing), stay under 15 per session.

Limitations

• - Catch-all domains accept all addresses; a "yes" doesn't guarantee a real inbox
• Some servers block SMTP verification (disconnect or timeout) — result will be "unknown"
• Greylisting temporarily rejects first attempts by design
• Rate limiting — don't bulk-verify hundreds from one IP; use a dedicated service for large lists
• Port 25 blocked — some ISPs/networks block outbound port 25; won't work from those environments
• Residential IPs may get flagged if used heavily — for bulk verification, prefer services like ZeroBounce or NeverBounce