SMTP Test Tool
See the exact SMTP conversation log to troubleshoot connection issues instantly.
Master Your Email Infrastructure
SMTP (Simple Mail Transfer Protocol) is the engine room of your email delivery. A successful “handshake” here is mandatory, but it’s only the first step. To ensure high Deliverability Rates, you must pair a healthy server connection with the correct ports, modern encryption, and a warmed-up sender reputation.
The SMTP Port Cheat Sheet
Choosing the wrong port is the #1 cause of connection timeouts. Use this matrix to select the correct configuration for your test.
| Port | Encryption | Use Case | Reliability Score |
|---|---|---|---|
| 587 | STARTTLS | Modern Standard. Best for almost all mail clients and APIs. | High |
| 465 | SSL / Implicit TLS | Legacy standard. Still used by older Microsoft/Outlook setups. | Medium |
| 25 | None / Unsecure | Server-to-Server only. Often blocked by ISPs to prevent spam. | Low (Avoid) |
| 2525 | STARTTLS | Alternative to 587. Use this if 587 is blocked by your firewall. | High |
Pro Tip: If your test times out on Port 587, try Port 2525. Many residential ISPs block standard mail ports.
3-Step Debugging Workflow
Authentication Check
Error 535 is the most common failure. It means your User/Pass is wrong. If you have 2FA enabled on your email account, you often need an “App Password” instead of your normal login password.
Firewall & Allowlisting
If the log says “Connection Refused,” your hosting provider might be blocking outbound traffic. Whitelist the IP address or try a non-standard port like 2525.
Protocol Mismatch
Don’t mix protocols. If you select SSL, you generally must use Port 465. If you select TLS, use Port 587. Mixing them (e.g., SSL on 587) will cause a handshake failure.
From Connection to Inbox
A successful SMTP test only proves your server can send. It does not prove that Google or Outlook will accept the email.
The “Relay Denied” Trap
If you see Error 550, your server is refusing to route email for your domain. This often happens if you haven’t authenticated the domain in your DNS records (SPF/DKIM).
Reputation > Connection
You can have a perfect SMTP setup and still land in Spam. Why? Sender Reputation. If your IP is cold, ISPs will throttle your traffic regardless of your technical setup.
The Missing Piece: Server Warmup
Technical setup is binary (it works or it doesn’t). Deliverability is fluid. Once your SMTP is verified:
- Authenticate: Ensure SPF, DKIM, and DMARC records match the SMTP server you just tested.
- Warm Up: Connect this SMTP account to a warmup service to gradually increase sending volume and build trust with ISPs.
Your SMTP is Ready. Is Your Reputation?
Don’t burn your new SMTP server with a sudden blast of emails. Warm it up automatically.
Warmup Inbox sends and replies to emails on your behalf to build a high-trust sender score.
Start Automating Your DeliverabilityFrequently Asked Questions
What is this SMTP Test Tool?
This is a web-based diagnostic tool that replaces manual command-line checks (like Telnet). It attempts to connect to your SMTP server, authenticate, and send a test payload, showing you the exact “conversation log” between our server and yours.
Is it safe to enter my SMTP password?
Yes. We do not store, log, or save your credentials. Your username and password are passed ephemerally to the SMTP server solely to perform the handshake test and are discarded immediately after the session closes.
Which port should I use?
For modern secure connections (TLS/STARTTLS), use Port 587. For legacy SSL connections, use Port 465. Avoid Port 25 as it is often blocked by ISPs. If 587 fails due to a firewall, try Port 2525.
What does “Error 535: Authentication Failed” mean?
This usually means your Username or Password is incorrect. If you have 2-Factor Authentication (2FA) enabled on your email account (like Gmail or Outlook), you cannot use your regular password; you must generate and use a specific “App Password.”
Can I test IP-based authentication without a password?
Yes. Simply leave the Username and Password fields blank. Ensure that your SMTP server is configured to whitelist/accept connections from the IP address this tool is running on.
Why did the test fail even though my credentials are correct?
Check the log. If you see “Connection Refused,” your firewall is blocking us. If you see “550 Relay Denied,” your server doesn’t recognize your domain. Also, ensure you aren’t mixing protocols (e.g., using SSL on a TLS port).