SMTP Banner Probe
Probe an SMTP server's banner, EHLO capabilities, and STARTTLS — or enter a domain to grade every MX host (PTR, open-relay, timing).
Email down = revenue down. Know before your customers do.
Monitor your mail server 24/7
- ✓SMTP / POP3 / IMAP monitoring
- ✓Port connectivity checks
- ✓Response time tracking
- ✓Email + Slack + SMS alerts
- ✓Incident history
- ✓Status page integration
Frequently Asked Questions
What does the SMTP Banner Probe check?
It connects to an SMTP server on port 25 or 587, reads the greeting banner, sends an EHLO command to list the server's capabilities, and tests whether STARTTLS is available and negotiates successfully. Results show the full banner, EHLO extensions (AUTH methods, size limits, etc.), and TLS negotiation status.
What is STARTTLS and why is it important?
STARTTLS upgrades a plain-text SMTP connection to an encrypted TLS connection before any credentials or message content are transmitted. Without it, email between servers can be intercepted in transit. Most modern mail servers require STARTTLS; its absence is a serious security issue and may fail compliance audits.
What does the EHLO response tell me?
EHLO lists the SMTP extensions the server supports: STARTTLS (TLS upgrade), AUTH LOGIN PLAIN (authentication methods), SIZE (max message size), 8BITMIME (8-bit content), SMTPUTF8 (international addresses). These affect how clients and other mail servers should connect and authenticate.
Why can't I probe port 25 from my own network?
Most ISPs and cloud providers block outbound port 25 to prevent spam. This tool probes from our infrastructure where port 25 is open. For testing user submission, try port 587 (SUBMISSION with STARTTLS) or 465 (SMTPS with implicit TLS), which are typically open.