Email Verifier

Verify email address deliverability via SMTP without sending mail. Checks MX records, performs RCPT TO verification, and detects catch-all domains. Use when validating email lists, checking if an email address exists before sending, cleaning lead lists, or verifying contact information. Supports single emails, batch verification, and CSV input.

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. 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

bash

pip3 install dnspython

Usage

Single or multiple emails

bash

python3 scripts/verify_email.py [e-mail] [e-mail]

From stdin

bash

echo "[e-mail]" | python3 scripts/verify_email.py --stdin

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

bash

python3 scripts/verify_email.py --csv leads.csv --email-column "Contact Email"

Options

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

Output

JSON array to stdout. Each result contains:

json

{
  "email": "[e-mail]",
  "domain": "example.com",
  "mx_host": "aspmx.l.google.com",
  "smtp_code": 250,
  "smtp_response": "2.1.5 OK",
  "deliverable": "yes"
}

Deliverability values

Value	Meaning
yes	Server accepted the recipient
no	Server rejected the recipient (invalid)
catch-all	Server accepts all addresses — cannot confirm inbox exists
unknown	Could not determine (timeout, block, greylisting)

Rate Limiting

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

bash

# Defaults: 1s between checks, max 20 per domain before 30s pause
python3 scripts/verify_email.py --csv leads.csv --email-column "Contact Email"

# Conservative: slower checks, lower burst limit
python3 scripts/verify_email.py --delay 3 --max-per-domain 10 --burst-pause 60 [e-mail]

# Aggressive (not recommended from residential IPs)
python3 scripts/verify_email.py --delay 0.5 --max-per-domain 50 [e-mail]

Options

• --delay SECONDS — Pause between each check (default: 1.0)
• --max-per-domain N — Max checks to one domain before pausing (default: 20)
• --burst-pause SECONDS — 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

Scenario	Recommended settings
Quick spot check (1-5 emails)	Defaults are fine
Small lead list (10-50 emails)	--delay 2 --max-per-domain 15
Larger batch (50-200 emails)	--delay 3 --max-per-domain 10 --burst-pause 60
Bulk verification (200+)	Use a dedicated service (ZeroBounce, NeverBounce) instead

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