smtp.banner.invalid_fqdn

Invalid Hostname in SMTP Banner

What we check

We parse the SMTP banner (the 220 greeting line) and check whether the hostname it presents is a valid fully qualified domain name (FQDN). RFC 5321 requires the banner to contain the server's FQDN.

What this finding means

The SMTP banner contains a hostname that is not a valid FQDN — it may be a bare hostname without a domain part (e.g., localhost, mail), an IP address without brackets, or a syntactically invalid string. This violates RFC 5321 §4.1.1.1.

Why it matters

RFC non-compliance. The SMTP greeting MUST contain a valid FQDN. Receivers may use the banner hostname for logging, diagnostics, or heuristic filtering. An invalid FQDN signals a misconfigured server.
Deliverability impact. Some anti-spam systems score against invalid FQDN banners. While not a common rejection reason, it contributes to negative heuristics.
Diagnostic value. When operators troubleshoot mail delivery, the banner is the first thing they see. A bare hostname like localhost makes it harder to identify the server.

How to fix

Set the banner hostname. For Postfix:
```
# /etc/postfix/main.cf
smtpd_banner = mail.example.com ESMTP
```
The hostname should be the server's public FQDN — the same name used in MX records and PTR records.
Reload Postfix:
```
postfix reload
```

Verify the banner:

telnet mail.example.com 25
# Expected: 220 mail.example.com ESMTP ...

How it's graded

An invalid banner FQDN carries a 10-point penalty in the MX category. See Grading Methodology for the full scoring model.

Evidence example

S: 220 localhost ESMTP Postfix
     ^^^^^^^^^ not a valid FQDN — missing domain part

References

RFC 5321 — Simple Mail Transfer Protocol