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.
It runs EHLO, STARTTLS, MAIL FROM and RCPT TO and shows you the full transcript, including handshake timing. Which command draws the error is what narrows the diagnosis: connection means an IP-level block, MAIL FROM means the sender domain, RCPT TO means the address itself.
It never AUTHs and never sends DATA. AUTH would mean handling credentials on a public tool; DATA would make this a way to send mail, which is an obvious abuse surface. The four commands it does run cover the overwhelming majority of real diagnostic cases.
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.