SMTP test of stripe.com.
Check that a mail server answers on its sending ports, encrypts the connection, and offers sign-in only once it is encrypted.
Looking up…
What the test does.
- On port 587 it connects, waits for the
220greeting, sendsEHLO, asks forSTARTTLS, completes the TLS handshake (which checks the certificate against the host name) and sendsEHLOagain. - On port 465 TLS starts with the first byte, then the same greeting and
EHLO. - It ends with
QUIT. It never sendsAUTH, a password or a message.
Reading the conversation.
Lines marked C: are what Hivex sent, S: what the server answered. Codes starting with 2 mean success (220 ready, 250 fine), 4 a temporary refusal and 5 a permanent one. After EHLO, each line names a feature: SIZE the largest message, AUTH the sign-in methods, PIPELINING, 8BITMIME and SMTPUTF8 how the server handles traffic and text.
What good looks like.
- Port 587 offers STARTTLS, and AUTH appears only in the answer after TLS.
- Port 465 completes TLS with a certificate for the exact name you use in your mail app.
- The greeting arrives within a second or two, and no step answers with a code starting with 4 or 5.
Questions people ask.
How do I test my SMTP server?
Type its host name, such as smtp.example.com. The test connects on ports 587 and 465, reads the greeting, switches to TLS and lists what the server offers, without ever signing in.
What are SMTP ports 25, 465 and 587 for?
Port 25 carries mail between mail servers. Port 587 is where mail apps send through their provider, upgrading to TLS with STARTTLS. Port 465 does the same with TLS from the first byte. RFC 8314 asks servers to offer both 587 and 465.
Why does my SMTP connection time out?
The port is closed or filtered, by the server's firewall or by your own network or internet provider. Many providers block outgoing port 25 for home connections, which is why mail apps use 587 or 465.
What is STARTTLS?
A command that turns a plain connection into an encrypted one before anything private is sent. On port 587 a server should offer it, and offer sign-in only after it.
Can this test port 25?
No. Hivex runs on Cloudflare Workers, which cannot open connections on port 25. To see how other servers reach a domain's mail, check its MX records, reverse DNS and MTA-STS policy.
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Sources
- RFC 5321: Simple Mail Transfer Protocol
- RFC 6409: Message submission for mail (port 587)
- RFC 3207: SMTP service extension for secure SMTP over TLS (STARTTLS)
- RFC 8314: Cleartext considered obsolete: TLS for email submission and access (port 465)
- RFC 4954: SMTP service extension for authentication (AUTH)
- Cloudflare: TCP sockets in Workers (no outbound port 25)
By Hivex. Updated 10 October 2026. Each test opens two short connections from Cloudflare, reads what the server says and signs off; nothing is stored, and no password is ever asked for or sent.