Question 1

Do you support testing IPv6-only mail servers?

Accepted Answer

Yes. SMTPTester resolves both A and AAAA records and connects via IPv6 when the host has no IPv4 address. Most modern providers (Gmail, Microsoft 365, SendGrid) advertise both.

Question 2

Can I use SMTPTester to verify a recipient address exists?

Accepted Answer

We deliberately do not send MAIL FROM / RCPT TO probes. Many mail servers treat probe traffic as abusive and will blocklist your IP. For recipient verification, use a dedicated email-validation API.

Question 3

Is there an API or CLI version?

Accepted Answer

Not yet — but it's on our roadmap. If you'd find one valuable, drop us a note via the contact page so we can prioritise.

Question 4

How is SMTPTester different from telnet?

Accepted Answer

Telnet only opens a raw TCP socket. SMTPTester walks the full ESMTP handshake, negotiates TLS, parses server capabilities, and explains each response in plain English — everything you'd build by hand on top of telnet, ready to go.

Question 5

What is an SMTP tester and how does it work?

Accepted Answer

An SMTP tester is a diagnostic tool that opens a real TCP connection to your mail server, performs the SMTP handshake (EHLO), optionally negotiates TLS, and attempts to authenticate using credentials you supply. SMTPTester returns the raw server response codes so you can pinpoint exactly where a problem occurs — connection, encryption, or authentication.

Question 6

Is it safe to enter my SMTP password?

Accepted Answer

Yes. Credentials are sent to our server over HTTPS, used only to perform the single test you requested, and are never written to logs, databases, or analytics systems. For production accounts we still recommend creating a dedicated SMTP user or app password.

Question 7

Which SMTP ports should I use?

Accepted Answer

Use port 587 with STARTTLS for modern submission, port 465 for implicit SSL/TLS, and port 25 only for server-to-server relay. Port 25 is widely blocked by ISPs and cloud providers for outbound mail.

Question 8

Why does my Gmail SMTP test fail with 535-5.7.8?

Accepted Answer

Google no longer accepts your regular password for SMTP. Enable 2-Step Verification and create an App Password at myaccount.google.com/apppasswords, then use that 16-character code as your SMTP password.

Question 9

Does a successful test guarantee inbox delivery?

Accepted Answer

No. The test confirms your server accepts the connection and credentials, but inbox placement also depends on SPF, DKIM, DMARC, sender reputation, content, and recipient filters. SMTPTester surfaces deliverability recommendations alongside each result.

Question 10

Can I test my own self-hosted Postfix or Exim server?

Accepted Answer

Yes. Enter the public hostname and port. Make sure the server is reachable from the public internet and that your firewall allows inbound connections on the SMTP submission port you are testing.

Question 11

What is the difference between SSL, TLS, and STARTTLS?

Accepted Answer

SSL/TLS (port 465) wraps the entire SMTP session in encryption from the first byte. STARTTLS (port 587) starts in plaintext, then upgrades to an encrypted channel via the STARTTLS command. Both deliver the same protection when configured correctly; STARTTLS is the modern recommended approach.

Question 12

Why is my SMTP test timing out?

Accepted Answer

Connection timeouts almost always indicate a network or firewall problem: the target host is unreachable, the port is blocked, or your provider blocks outbound SMTP. Try a different port (587 vs 465) or run the test from a different network.

Frequently asked questions