Probe any SMTP server live — see the full wire transcript.
Enter a host + port + sender address. We open a raw socket, run the SMTP handshake one verb at a time, and stream the transcript back. Never sends DATA — purely a diagnostic, so this can't be used to actually deliver mail.
The classic "why isn't my mail sending" debugging session is mostly an SMTP probe in disguise. Is the server even reachable? Does the banner come back? Does STARTTLS upgrade cleanly or is there a TLS cert mismatch? Does the server accept the MAIL FROM address? Does it know about the RCPT TO? Each of those is a separate failure mode and the transcript shows you exactly which one.
STARTTLS upgrades are where most subtle deliverability problems hide. A mail server that advertises STARTTLS but fails the handshake because of a self-signed cert or expired chain falls back to plaintext on some clients and fails outright on others — and the failure is silent in the application logs. The transcript here shows the handshake timing explicitly.
The tool deliberately never AUTHs and never sends DATA. AUTH would require credentials we don't want to handle on a public free-tool; DATA would let the tool be used to send actual mail, which is an obvious abuse surface. EHLO / STARTTLS / MAIL FROM / RCPT TO covers ~95% of real diagnostic cases — TLS misconfig, port closed, banner refusing connections from your IP, recipient rejected.
Not from the free tool — we don't want to handle credentials. The Inbox Placement add-on inside SieveGuard runs full authenticated SMTP tests on every mailbox you connect.
So this can't be used to send actual mail. The diagnostic value is in EHLO / STARTTLS / MAIL FROM / RCPT TO — those four cover almost every "why isn't my mail sending" question.
587 for modern submission (most ESPs), 465 for implicit TLS (older but still common), 25 for server-to-server. 25 from your laptop is usually blocked by your ISP.
Residential ISPs block outbound 25 to slow botnet-spam. From your laptop, test against port 587 instead. From a server, port 25 should work if your hosting provider hasn't also blocked it (some do).
On port 587 / 465, yes — modern mail servers expect TLS. Port 25 server-to-server is allowed to fall back to plaintext if STARTTLS isn't advertised, though most major receivers now expect it.
SieveGuard's Mailbox Warmup add-on runs authenticated EHLO / AUTH / send tests on every mailbox you connect, daily, with full deliverability scoring per ESP. Free during your trial.