SMTP Tester: what it does and how to use it

Find out why mail is not going out. This tester speaks SMTP to the server exactly as a mail client would: it opens the connection, reads the greeting banner, sends EHLO, upgrades the session with STARTTLS where it is offered, inspects the TLS certificate, and - if you supply credentials - checks that authentication is accepted. Every step is timed and shown with the actual server reply, so instead of a vague failure you get the precise point where things break. It deliberately cannot send email, which keeps the endpoint from being usable as a spam relay.

SEO and webmaster checks are only useful if they match what Googlebot or a browser would see. Guessing from the HTML source alone is how people ship broken tags.

SMTP Tester is a good fit when working out why an application cannot send email through a provider.

The useful part

SMTP Tester is built around a few practical wins, not a long feature list:

• A step-by-step trace with real server replies, rather than a single pass or fail.
• TLS certificate subject, issuer and expiry, with a warning when it is close to expiring.
• Lists the AUTH mechanisms and capabilities the server advertises after the TLS upgrade.
• Re-reads capabilities after STARTTLS, which is where AUTH usually appears - checking before the upgrade reports it as missing.
• Cannot send email by design, so it is safe to run against your own production server.
• Credentials are used for one session and never logged or stored.

Do this, in order

1. Enter the server and port. Use 587 for modern submission with STARTTLS, 465 for implicit TLS, or 25 for server-to-server relay.
2. Optionally add a login. Supplying a username and password adds an AUTH check. They are used for that one connection and never stored.
3. Read the trace. Each step shows whether it succeeded, how long it took and what the server actually replied - which is usually where the problem is obvious.

Who it is for

• Working out why an application cannot send email through a provider.
• Confirming that a new mail server is reachable and negotiating TLS before going live.
• Checking whether an app password has been accepted after rotating credentials.
• Verifying that a certificate renewal actually took effect on the mail service.
• Deciding whether to use port 587 with STARTTLS or 465 with implicit TLS.
• Confirming a firewall rule opened the submission port from the outside.

If you want a clean result

• Use an app password rather than your main account password wherever the provider offers one.
• Port 465 is always implicit TLS; the encryption selector follows the port automatically.
• If STARTTLS is not advertised, the server may only accept encryption on a different port.
• A 535 reply to AUTH means the credentials were rejected, not that the server is broken.
• Certificate expiry inside 30 days is flagged - renew before it bites.

Common mix-ups

• Expecting a test email to arrive - this tool stops at authentication and never sends mail.
• Testing port 465 with STARTTLS selected; 465 expects TLS from the first byte.
• Reading no AUTH mechanisms as a fault when the server only advertises them after STARTTLS.
• Using a main account password where the provider requires an app-specific one.
• Assuming a successful login means delivery will work - SPF, DKIM and DMARC are separate matters.

Private by default

SMTP Tester needs the ToolBox API for work a browser cannot do. What you submit is processed, then discarded. It is not kept as a library of your files.

Related tools worth opening next

If this is one step in a longer job, these usually come after it:

• Reverse IP Lookup - Find the hostname behind an IP address - the PTR record mail servers and firewalls check
• Ping Tool - Measure latency, jitter and packet loss to any server - from our network, not yours
• Web Page Scraper - Pull the title, metadata, headings, links, images and text out of any public page

Open the SMTP Tester when you are ready. It is free, and you do not need an account.