SMTP Handshake Test
SMTP Test validates safe handshake flow with a target mail server without sending actual email. It is useful for checking banner quality, EHLO support, and TLS negotiation behavior.
π§ͺ Run this tool π Excessive Explanation β Frequently asked questions
π§ͺ Run this tool
π Excessive Explanation
𧩠Technical Details
What this tool checks
- SMTP greeting and capability response behavior.
- Port-specific handshake stability for 25, 465, and 587.
- STARTTLS upgrade signal and transcript interpretation.
How to read the output
- Result Summary highlights whether handshake path is operational.
- Overview helps identify port-specific failures quickly.
- Technical Details contains transcript context for protocol debugging.
- Raw Output supports ticketing with mail providers and hosters.
Common failure patterns
- Banner timeout due to overloaded or filtered SMTP listener.
- TLS negotiation fails on one port but not others.
- STARTTLS advertised but handshake upgrade breaks.
- Incorrect submission port policy after provider changes.
Remediation workflow
- Confirm correct host and port for intended SMTP role.
- Validate cert chain and TLS policy on submission and relay ports.
- Check upstream firewall/NAT rules for SMTP traffic.
- Re-test each port after policy updates.
Next steps
β Frequently asked questions
Does this tool send real email messages?
No. It performs handshake diagnostics only.
Why can 587 work while 25 fails?
Submission and relay paths often have different policy, firewall, and anti-abuse controls.
Is port 465 always better?
It depends on provider and client requirements; test against your actual deployment policy.
Can STARTTLS failure affect deliverability?
Yes. Some receiving systems and compliance policies require successful TLS negotiation.
What should I check after SMTP handshake success?
Validate sender auth records and reputation posture.