What you can learn
Each check is a reply code or a DNS record
The result is a short list of observations from this session. Read the label beside the evidence. A pass means that step succeeded here. It does not mean every future message will arrive.
Connection
The banner is the first line the server sends, usually a 220. If the TCP connection fails, the label is Unreachable and later SMTP steps stay not tested.
TLS
STARTTLS upgrades after EHLO. Implicit TLS encrypts from the first byte. If you request either one and it fails, the label is TLS failed and the password is not sent.
Authentication
One login attempt. A 235 is labeled Authenticated, which means this server accepted the credentials. A 535 is Authentication failed. The test does not retry.
Message acceptance
If you add a recipient, the tester offers one small message. Accepted by this server means MAIL, RCPT, and DATA each returned 250. It does not prove the mailbox displayed it.
MX, SPF, and DMARC
These rows use the From domain. SPF uses the result names from RFC 7208 and is separate from the SMTP session. DMARC shows the published policy record.
DKIM
DKIM stays not tested until you enter a selector. A published DNS key is labeled DNS key present. That record does not prove a signature on a message will pass.
How to use
How to test an SMTP server
- Pick your provider to fill the SMTP host, port, and encryption, or copy them from your mail settings. Port 587 with STARTTLS is the usual choice. Port 465 uses implicit TLS.
- Add the username and password only when you want to test login. The password is used once and is not stored.
- Enter the From address you send as. MX, SPF, and DMARC are looked up for that domain.
- Add a To address to send one test email through your server. Leave it blank to check only the connection, TLS, and login. TechMamba is not the mail relay.
- Read each row next to the reply code or DNS record. Authenticated means 235. Accepted by this server means MAIL, RCPT, and DATA returned 250.
Ports
Choose the port your provider published
Start with the settings in your mail panel. The tester allows 25, 465, 587, and 2525, plus other high ports. It refuses private addresses and common internal service ports, so the form is a mail check, not a network scan.
587 with STARTTLS
The usual submission setup for apps and websites. The connection starts in the clear, then upgrades when the server advertises STARTTLS.
465 with implicit TLS
Encryption is required from the first byte. Choose Implicit TLS in the form when the provider lists SSL or TLS on port 465.
25 for server mail
Port 25 is for mail between servers and is often blocked on website hosts. Use it only when the provider says that is the correct port.
Read the labels
What each result means
- Unreachable
- The TCP connection failed before a useful SMTP conversation.
- TLS failed
- EHLO succeeded, and the TLS step you requested did not. The password is not sent.
- Authentication failed
- The server returned 535 for this username and password. The test makes one attempt.
- Authenticated
- The server returned 235. The credentials were accepted. That does not prove inbox delivery.
- Accepted by this server
- MAIL, RCPT, and DATA each returned 250. This server took the message. It does not prove the mailbox displayed it.
- No reply after DATA
- The message was sent, but the server did not answer within 30 seconds. Some providers scan mail first, so check the mailbox before treating it as a failure.
- Not tested
- That step was skipped, often because an earlier step failed or you left the field blank.
Who it is for
One check the whole team can read
Developers
Confirm the host, port, encryption, and login your application uses before you ship a contact form or notification.
Marketers
See whether the From domain publishes MX, SPF, and DMARC, and whether a DKIM selector has a DNS key, without a spam score.
Founders
Check a new domain or a new mailbox before customers rely on invoices, receipts, and password resets.
Agencies
Show a client the reply codes and DNS records from one session, instead of a number that claims more than the test observed.
SMTP Tester FAQ
Is the SMTP tester free?
Yes. The TechMamba SMTP tester is free and does not require an account. It opens one socket connection from the TechMamba server to the mail host you enter, then looks up DNS with PHP. It does not call a paid mail-testing API, and it does not send the message through TechMamba mail.
Who should use this SMTP test?
Use it when a website contact form, app notification, invoice, password reset, or new domain is not sending mail. Developers can confirm the host, port, TLS, and login. Marketers and founders can check the From domain DNS. Agencies can show a client the same reply codes instead of a vague deliverability score.
Does a successful test mean the message reached the inbox?
No. A 235 reply means this server accepted the username and password. A 250 reply on MAIL, RCPT, and DATA means this server accepted the message. Neither reply proves that a mailbox displayed the message, and the tester does not assign an inbox or spam label.
Do you store the SMTP password?
No. The password is used for one AUTH attempt during the request and is not stored, logged, or returned. The form clears it after you submit, and the SMTP transcript replaces the AUTH payload with a redaction marker.
What do the result labels mean?
Unreachable means the TCP connection failed. TLS failed means EHLO succeeded and the requested TLS step did not. Authentication failed is the label for a 535 reply. Authenticated is the label for a 235 reply. Accepted by this server is the label when MAIL, RCPT, and DATA each return 250. SPF uses the RFC 7208 result names and is reported separately from the SMTP session.
Why is there no spam or blacklist score?
An SMTP session cannot tell you whether a mailbox will file a message as spam. Public blacklist queries are also a poor fit for a web server, because they are often blocked or incomplete. The tester shows the reply text and DNS record instead of a score that would overstate what was observed.
Which port should I use, 587, 465, or 25?
Start with the settings your provider published. Port 587 with STARTTLS is the common submission setup. Port 465 uses implicit TLS, where the connection is encrypted from the first byte. Port 25 is for server-to-server mail and is often blocked, sometimes by the network running this test, so Unreachable on port 25 does not always mean your server is down. The tester also allows port 2525 and other high ports, and it refuses common internal service ports.
What is the difference between STARTTLS and implicit TLS?
STARTTLS starts as a plain connection, then upgrades after EHLO if the server advertises it. Implicit TLS, sometimes called SSL on port 465, expects encryption immediately. If you request either one and the upgrade or handshake fails, the label is TLS failed and the password is not sent.
Why did authentication fail with a 535 reply?
A 535 reply means this server rejected the username or password on this attempt. Check the mailbox username, an app password if the provider requires one, and that SMTP AUTH is enabled. Some providers also refuse logins from a web server even when the password is correct. The tester makes one attempt and does not retry.
What does Accepted by this server mean?
It means the SMTP server you tested returned 250 for MAIL, RCPT, and DATA. That server took the small test message in this session. It does not prove the recipient mailbox displayed it, and it does not mean TechMamba delivered the mail for you.
Does this tool check SPF, DKIM, and DMARC?
It looks up MX, SPF, and DMARC for the From domain and shows the DNS evidence separately from the SMTP replies. SPF uses the result names from RFC 7208. DKIM stays not tested until you enter a selector. A published DKIM DNS key does not prove a signature on a message will pass.
Can I test Gmail, Outlook, Microsoft 365, or my own server?
You can test a public SMTP hostname you are allowed to use, including a provider host or your own mail server. The host must resolve to a public address. Private, loopback, and reserved addresses are refused. A provider may still block or limit SMTP AUTH from a website, so read the reply text before treating a failure as a bad password.