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 Deliverability