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, and in how much detail each tool shows you along the way.
Who each tool suits
smtper.net fits the "send one message and see if it lands" workflow. You enter a host, port, credentials, and message content, hit send, and get back whether the mail went through. If all you need is that yes/no answer, it does the job with minimal setup.
SMTP Tester fits the same workflow plus everything that tends to follow it: figuring out why a server refused the message, verifying that TLS actually negotiated cleanly, checking credentials on a production MTA without queuing real mail, or handing evidence to a teammate or support team. It reports the full protocol conversation rather than only the final verdict.
In practice: if you test the same SMTP server regularly, troubleshoot deliverability or authentication problems, or manage MTAs like KumoMTA or PowerMTA, the diagnostic layer earns its keep. If you test a personal mail account once a year, a simple sender is enough.
What a test looks like in each tool
During the test
With a simple sender, you typically watch a spinner and wait. The connection, encryption, authentication, and message submission all happen behind the scenes, and you see the end result when they finish.
SMTP Tester streams the SMTP conversation as it happens. Each line the client sends and each response the server returns appears live, grouped into stages: connect, TLS/STARTTLS, EHLO, AUTH, MAIL FROM, RCPT TO, and DATA. A stage timeline shows where the test currently is and which stages passed. When something stalls or fails, you see the exact command and the exact response code, not a summary generated after the fact.
After the test
A basic sender reports success or a short error message. That is often enough when the test passes and often not enough when it does not, because "535 authentication failed" with no surrounding context tells you nothing about whether the connection itself, the TLS upgrade, or the credentials were at fault.
After a SMTP Tester run you have:
- The complete, credential-redacted transcript, filterable and copyable as text or JSON.
- Per-stage timings and status codes (250, 235, 354, and so on).
- TLS details from a separate probe.
- A short fix suggestion with a link to the matching guide when a known error appears, such as SMTP authentication failed.
You can paste that transcript into a support ticket or a pull request review and the recipient can see precisely what the server said.
What smtper.net is good for
To be fair to the simpler tool: a no-frills sender has real advantages. There is less to configure, less output to read, and for a single send test the extra stages and panels of a diagnostic tool are unused. smtper.net is a straightforward browser-based way to confirm a server accepts a message, and if that is the whole task, it completes it.
The trade-off appears when the answer is "no". Without a transcript you cannot tell whether the server rejected the AUTH line, refused the relay attempt at RCPT TO, or dropped the connection during STARTTLS, so the next step usually becomes manual debugging with telnet anyway.
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. If you are testing a provider relay such as Gmail's SMTP server, the probe also confirms which TLS version the provider negotiated for your connection.
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. It is the closest browser-based equivalent to a swaks handshake check, and it avoids any risk of a test message reaching a live customer or a real recipient.
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. This matters more than it sounds: a raw AUTH PLAIN line contains base64-encoded credentials, and pasting an unredacted transcript anywhere public leaks the account.
Saved profiles and rotating test emails
Connection profiles are stored in your browser's local storage (password only if you explicitly opt in), so repeat tests against the same server need no re-typing. When a message is sent, SMTP Tester uses a rotating set of professionally designed templates rather than a bare plaintext mail, which gives you a more realistic check of how the message renders.
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 |
| Per-stage status codes and timings | 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 |
| Rotating designed test emails | No | 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.
That model matters when you test shared or customer credentials: nothing about the test persists on the tool's side, and the redacted transcript is the only artifact you carry away.
How to choose
Match the tool to the question you are actually asking:
- "Does this account send mail at all?" Either tool answers this. Pick whichever you reach for first.
- "Why does this server reject mail?" Use SMTP Tester. The live transcript and per-stage status codes isolate the failing step (connection, TLS, AUTH, or relay).
- "Is the TLS setup healthy?" Use SMTP Tester's TLS probe, which shows protocol, cipher, and expiry that a send-only test cannot.
- "Does this MTA accept the auth method I configured?" Use SMTP Tester and pick PLAIN/LOGIN/CRAM-MD5 explicitly. This is the common case when standing up KumoMTA or PowerMTA.
- "Can I verify production credentials without sending mail?" Use SMTP Tester's handshake-only mode.
- "Do I need to share test evidence?" Use SMTP Tester's redacted transcript or JSON export.
A reasonable setup is to keep a simple sender around for quick checks and use SMTP Tester whenever a test fails, when credentials are shared, or when the server is anything more exotic than a standard mailbox provider. The provider-specific guides in this library — Outlook 365, SendGrid, Amazon SES, and others — work with either tool, but the error-code tables in them map most directly to transcript output.
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.
Start with handshake-only mode if the server is in production, then switch to a full send test once the handshake is clean.
Where to go next
- What is an SMTP test? — the concepts behind both tools
- SMTP port 587 vs 465 — picking the right port and encryption mode
- Gmail SMTP test — testing Google's relay, a common first target
- SMTP authentication failed — the error you will meet most often
Frequently asked questions
Is SMTP Tester free like smtper.net?
Yes. Both tools are free to use from the browser without an account. SMTP Tester is also open source, so you can read the code or self-host it if you prefer your credentials never to touch a third-party service.
Do I need to install anything with either tool?
No. Both run entirely from the browser. The traditional alternative, a telnet or openssl s_client session, requires a terminal, knowledge of SMTP commands, and an unblocked outbound route to your server.
Can either tool read my password?
SMTP Tester transmits your credentials only to the SMTP server you specify, does not store them server-side, and redacts them from the transcript. Saved profiles stay in your browser's local storage. smtper.net also does not keep passwords on a server, per the comparison above.
Why does the transcript matter if my test fails?
Because the fix depends on which stage failed. A 535 at AUTH means credentials or the auth mechanism; a 550 at RCPT TO means the server does not relay for you; a TLS alert during STARTTLS means a certificate or port problem. The transcript tells you which one happened, and SMTP Tester links each common code to a matching guide with the fix.
Can I test without sending an email?
With SMTP Tester, yes, using handshake-only mode: it connects, negotiates TLS, and authenticates, then stops before MAIL FROM. A simple sender always submits a message as part of its test.
Does SMTP Tester work with provider relays like Gmail or SendGrid?
Yes. Any server that speaks SMTP over 465, 587, or 25 works, and the TLS probe plus transcript are equally useful against large providers. The provider guides in this library list the correct host, port, and security mode for each one.