dotvitals

Mail server's SMTP greeting does not conform to the protocol

LowConfirmedQuick winemail.smtp-tls.banner-malformed

What this check looks for

Your mail server's opening greeting is not shaped the way the standard requires — it is over-long, contains characters it should not, or does not start with a reply code. Strict senders may refuse to continue.

Why it matters

A malformed greeting is usually a misconfigured banner or a device in front of the server rewriting traffic. Some senders abandon the connection, and a greeting carrying control characters can corrupt whatever logs or displays it.

When the check passes, your report says: “The SMTP greeting is shaped the way the standard requires”.

What it costs your score

When this check fails it removes 3 points from your Email transport score, before the status, confidence and repeat multipliers are applied. Email transport carries a weight of 8 in the overall score.

It shares the email-transport.smtp-tls family ceiling of 40 points: however many findings that family produces, together they cannot remove more than that from Email transport. One underlying problem showing up in several places is still one problem.

Severity
low
Default confidence
confirmed
Status when triggered
warn
Deduction
3 points
Family cap
email-transport.smtp-tls · 40
Category
Email transport
Module
Email smtp tls
Fix owned by
mail provider
In the ruleset since
2026.09

How the whole score is calculated

How to fix it

Set a conforming SMTP banner on ‹mx host›.

A greeting that breaks the protocol can cause strict senders to abandon the connection.

  1. Set the banner to the server's own hostname and nothing surprising: Postfix smtpd_banner = $myhostname ESMTP.

  2. Keep it well under 512 characters and free of tabs, newlines and non-ASCII characters.

  3. If a firewall or mail gateway sits in front of the server, check whether it is rewriting the greeting.

How to confirm it worked

  • nc ‹mx host› 25 — the first line should read like 220 ‹mx host› ESMTP.

The configuration to publish
# Postfix (main.cf)
smtpd_banner = $myhostname ESMTP

A named slot like ‹domain› — and the braces left in the configuration below — is filled in with your own values when this rule appears on a report.

Remediation by platform

Postfix
smtpd_banner = $myhostname ESMTP
Technical detail

The greeting from ‹mx host› was ‹banner›. ‹banner issue› RFC 5321 §4.2 requires a reply to begin with a three-digit code, and §4.5.3.1.5 limits a reply line to 512 octets including the CRLF. dotvitals strips control characters and truncates before storing or displaying anything a mail server sends.

Standards and references

Test this on your domain

Run the check that produces this finding, on its own, against any domain.

Open the email smtp tls checker

Other email smtp tls checks