smtp.banner.hostname_mismatch
Banner Hostname Does Not Match MX Hostname
What we check
We compare the hostname in the SMTP banner (the 220 greeting line) with the MX hostname from DNS. RFC 5321 recommends that these match — the server should identify itself with the name that DNS points to it.
What this finding means
The hostname in the SMTP banner does not match the MX hostname that DNS resolved. For example, the MX record says mail.example.com, but the banner says 220 mx01.provider.net ESMTP. This is common with hosted mail services and reverse proxies where the underlying server has its own hostname.
Why it matters
- Diagnostic confusion. When troubleshooting delivery issues, the banner is the first identity check. A mismatch makes it harder to confirm which server is responding.
- Heuristic scoring. Some anti-spam filters use banner/MX consistency as one input to reputation scoring. A mismatch is not a rejection trigger by itself, but it contributes.
- Expected for hosted mail. If you use a mail hosting provider (Microsoft 365, Google Workspace, managed relay), the mismatch is normal and not actionable — the provider controls the banner.
How to fix
For self-hosted servers, set the banner to match your MX hostname. For Postfix:
# /etc/postfix/main.cf smtpd_banner = mail.example.com ESMTPThen reload:
postfix reloadFor hosted mail services, no action is needed. The provider sets the banner and it is expected to differ from your domain's MX hostname.
How it's graded
A banner hostname mismatch is informational and carries no penalty. See Grading Methodology for the full scoring model.
Evidence example
MX hostname: mail.example.com
SMTP banner: 220 mx01.mailprovider.net ESMTP
^^^^^^^^^^^^^^^^^^^^^^ does not match MX hostname