smtp.session.unavailable
SMTP Session Not Established
What we check
We attempt to connect to the mail server on port 25 and complete an SMTP session. This finding is emitted when no SMTP session could be established at all — the connection was refused, timed out, or the server did not respond with a valid SMTP banner.
What this finding means
The scanner could not establish any SMTP session with this IP address. No further SMTP-level checks (STARTTLS, relay, role addresses) could be performed. Common causes:
- Port 25 is blocked. A firewall, cloud security group, or hosting provider blocks inbound port 25 on this IP.
- The MX points to the wrong IP. The DNS MX record resolves to an IP that does not run an SMTP server.
- Server is down. The mail service is not running, crashed, or overloaded.
- Connection timeout. Network path issues prevent the connection from completing within the timeout window.
Why it matters
- No mail delivery. If no SMTP session can be established, senders cannot deliver mail to this MX. Messages will be deferred and eventually bounce.
- Per-IP impact. Because we probe every IP individually, this finding affects only the specific IP that failed. Other IPs for the same MX hostname may still be reachable.
- Cascading failures. If this is the only MX or the highest-priority MX, all inbound mail for the domain is affected.
How to fix
Verify port 25 is open. Check firewall rules and cloud security groups. Many cloud providers (AWS, Azure, GCP) restrict outbound port 25 by default, but inbound port 25 for MX servers must be explicitly allowed.
Verify the mail service is running:
systemctl status postfix ss -tlnp | grep :25Verify DNS is correct. Ensure the MX record points to an IP that actually runs a mail server:
dig +short MX example.com dig +short A mail.example.comTest from an external network. The server may be reachable from the local network but blocked from the internet due to firewall rules.
How it's graded
An unavailable SMTP session prevents all SMTP-level checks from running. The server receives no credit for any SMTP or TLS category tests. See Grading Methodology for the full scoring model.
Evidence example
Connecting to 203.0.113.10:25...
Connection timed out after 30s — no SMTP banner received.