modbender 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 user@example.com another@domain.com
From stdin
echo "user@example.com" | 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": "user@example.com",
"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@example.com
# Aggressive (not recommended from residential IPs)
python3 scripts/verify_email.py --delay 0.5 --max-per-domain 50 email@example.com
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