email-verifier
v1.0.1Verify 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 sin...
Sourced from ClawHub,
Authored by psyduckler
Installation
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
- MX Lookup — Resolves the domain's mail exchange server
- SMTP Handshake — Connects to the MX server on port 25
- RCPT TO Check — Asks the server if it would accept mail for the address
- Catch-All Detection — Tests a random address to detect catch-all domains
Dependencies
pip3 install dnspython
Usage
Single or multiple emails
python3 scripts/verify_email.py [email protected] [email protected]
From stdin
echo "[email protected]" | python3 scripts/verify_email.py --stdin
From CSV (e.g., a lead list)
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:
{
"email": "[email protected]",
"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:
# 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 [email protected]
# Aggressive (not recommended from residential IPs)
python3 scripts/verify_email.py --delay 0.5 --max-per-domain 50 [email protected]
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