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SMTP Transaction Test

Execute a full, simulated mail delivery transaction to diagnose MTA handshake errors.

STAGES: EHLO, MAIL FROM, RCPT TO, QUIT
FEEDBACK: Raw Server Banners
ENCRYPTION: STARTTLS Negotiation

What is SMTP Transaction Test?

Instead of simply checking if a port is open, this tool acts as a fully compliant Mail Transfer Agent (MTA) and talks directly to the target server, logging every single response code.

How it Works

The script connects to the target server, introduces itself, attempts to securely upgrade the connection, and verifies if the server rejects the sender domain or recipient format.

Key Use Cases

Debugging cryptic "554 Transaction Failed" error messages.

Verifying that greylisting filters are artificially delaying connections.

Testing if recipient validation (Directory Harvest Protection) is enabled.

Advanced Details

If a server responds with `4xx` error codes rather than `5xx` error codes during this test, it indicates a temporary network or rate-limiting issue, not a permanent block.

Cybersecurity Pros & Cons

Security Advantages
  • Captures the full SMTP banner, which often reveals the MTA software and version — critical for CVE vulnerability matching.
  • STARTTLS negotiation testing confirms whether opportunistic encryption is functional or silently failing.
  • Raw SMTP response codes directly expose misconfigured sender restrictions, authentication requirements, and relay policies.
  • Simulating a full MTA handshake reveals greylisting behaviour that causes email delays in enterprise environments.
Security Limitations
  • SMTP transaction testing generates connection logs on the target server — detectable by the target's security monitoring.
  • Servers with aggressive intrusion prevention may temporarily block the testing IP after repeated connection attempts.
  • Response codes alone do not confirm message delivery — a 250 OK to RCPT TO does not mean the message will reach the inbox.
  • SMTP banner grabbing reveals your scanning tool's identity to the target via the EHLO hostname string.
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