If you have used smtper.net to send a quick test email through an SMTP server, SMTP Tester does the same core job and adds deeper diagnostics. Both tools connect to the SMTP server you specify and can deliver a test message. This page compares them factually so you can pick the right tool for the task.
What both tools do
- Connect to any SMTP host and port you enter.
- Authenticate with a username and password.
- Send a test message and report whether the server accepted it.
- Run from the browser — no software to install locally.
For a one-off "does this server accept a message" check, either tool works. The differences show up when a test fails and you need to understand why.
Where SMTP Tester goes further
Live protocol transcript
SMTP Tester streams every client and server line of the SMTP conversation in real time — the same lines you would see from telnet or openssl s_client, including the EHLO capabilities, STARTTLS upgrade, AUTH exchange, and the final response after DATA. When a test fails, you see the exact step and status code that caused it instead of a single pass/fail message.
TLS certificate diagnostics
A separate TLS probe reports the negotiated protocol (TLS 1.2/1.3), cipher suite, certificate subject, issuer, and days until expiry. This helps confirm encryption actually succeeded and catches expiring or mismatched certificates that a plain send test would not surface.
Multiple authentication methods
SMTP Tester supports Auto-detect, AUTH PLAIN, AUTH LOGIN, and CRAM-MD5. This matters for MTAs with specific requirements — for example KumoMTA only accepts AUTH PLAIN, and PowerMTA uses password auth per its smtp-user configuration. You can also choose no authentication for trusted-network relay testing.
Handshake-only mode
You can run connect → EHLO → STARTTLS → AUTH without sending a message. This verifies connectivity, encryption, and credentials on a production server without generating real mail in its queue.
Credential redaction
Passwords and AUTH payloads are automatically redacted from the transcript before it is displayed or exported, so you can safely copy the output into a support ticket, forum post, or team chat.
JSON export and contextual hints
Results — including stages, timings, TLS details, and the transcript — can be exported as JSON. When a test fails, SMTP Tester maps common error codes (535 authentication failed, connection timeouts, TLS errors) to a short fix and a link to the relevant guide.
Feature comparison
| Capability | smtper.net | SMTP Tester |
|---|---|---|
| Send a test message | Yes | Yes |
| Live protocol transcript | No | Yes |
| TLS certificate diagnostics | No | Yes |
| Auth methods | Basic | PLAIN / LOGIN / CRAM-MD5 / auto |
| Handshake-only mode | No | Yes |
| Credential redaction in output | — | Yes |
| JSON export of results | No | Yes |
| Contextual error hints + guides | No | Yes |
| Stores passwords on the server | No | No |
| Saved connection profiles | No | Browser-local only |
This table reflects SMTP Tester's documented features. Third-party tools change over time — verify a competitor's current capabilities on its own site.
Privacy model
SMTP Tester has no database and no user accounts. The credentials you enter are used only to connect to your server for a single test and are never written to disk. Saved connection profiles live only in your own browser's local storage, and you choose whether to include a password. See the Privacy Policy for the full details.
Which should you use?
- Quick one-off send test: either tool is fine.
- Debugging why a server rejects mail: SMTP Tester's live transcript and per-stage status codes isolate the failing step (connection, TLS, AUTH, or relay).
- Verifying TLS/certificates: SMTP Tester's TLS probe shows protocol, cipher, and expiry that a send-only test cannot.
- Testing an MTA with specific auth (KumoMTA, PowerMTA): SMTP Tester lets you pick PLAIN/LOGIN/CRAM-MD5 explicitly.
- Production checks without sending mail: use handshake-only mode.
Try it on your own server
Enter your host, port, security mode, and credentials, then watch the live transcript. It is a no-install, browser-based alternative to debugging SMTP with telnet, openssl s_client, or swaks — and it works even on networks where outbound port 25 is blocked.