Blog article
- 8 minutes read
- Kiara Saloojee
- DMARC Enthusiast
Reverse DNS mismatch: Fix SMTP banner errors for reliable delivery
Reverse DNS can impact your deliverability, even when everything else looks fine. If you’re seeing errors that say reverse DNS does not match SMTP banner, it usually means your sending IP address points back to one hostname, while your email server introduces itself as another.
This mismatch is a trust problem. Receiving email servers use reverse DNS as a credibility signal during the SMTP connection. When reverse DNS and the SMTP banner are out of alignment, your messages can be rejected.
A useful analogy is caller ID. Reverse DNS is the caller ID for an IP address. If the server says “I’m mail.example.com,” but the IP’s reverse DNS says something else, the receiver may suspect misconfiguration or spoofing.
What is reverse DNS and why it matters for email
What is reverse DNS? Reverse DNS is a mapping process that links an IP address back to a hostname.
Reverse DNS is the counterpart to forward DNS:
- Forward DNS maps a hostname to an IP address (usually an A or AAAA record).
- Reverse DNS maps an IP address back to a hostname.
In email delivery, reverse DNS matters because many receivers perform a basic consistency check.
The receiving servers can compare:
- The connecting IP address’s hostname
- The hostname shown in the SMTP banner greeting
If these values don’t align, you may trigger warnings such as reverse DNS does not match SMTP banner, which can directly impact deliverability.
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Causes of “reverse DNS does not match SMTP banner”
Most reverse DNS mismatches come down to a small set of root causes.
Misconfigured DNS records
The PTR record may point to the wrong hostname, or the A record doesn’t resolve to the same IP address.
Non-updated SMTP settings
If the SMTP banner still shows a previous domain, the “reverse DNS does not match SMTP banner” error becomes almost inevitable.
Shared outbound IP addresses
In shared hosting, multiple domains often have the same IP address. Because an IP can only have one PTR record, a domain’s reverse DNS might not match another’s SMTP banner.
Steps to fix reverse DNS and SMTP banner mismatch
Step 1: Confirm the mismatch (banner vs. PTR)
Check what your email server announces in its SMTP greeting (banner). Then compare that hostname to the reverse DNS (PTR) result for the sending IP. If they differ, you’ve confirmed the issue: The greeting domain doesn’t match the PTR record.
Step 2: Review headers for clues
If you’re seeing bounces or delays, inspect the message headers and details. They often point to the exact hostname being rejected and help you confirm which server is actually sending.
Step 3: Fix reverse DNS and forward DNS alignment
Verify the PTR record maps to the same hostname you want to use in the SMTP banner. Then make sure that the hostname has an A record that resolves back to the same IP. Both forward and reverse lookups need to match cleanly.
Step 4: Update the email server identity
Adjust your email server settings so the hostname shown in the SMTP banner matches the hostname in your PTR (and A) records. If these aren’t aligned, receivers treat the configuration as untrustworthy.
Step 5: Verify and validate with real sends
Send test messages to providers like Gmail, Outlook, and Yahoo, ensuring the “reverse DNS does not match SMTP banner” error is gone. Finally, send an email to yourself and review the full header to ensure everything is aligned.
Prevent future reverse DNS failures and protect deliverability
Reverse DNS issues often resurface after infrastructure changes, so include reverse DNS checks in every email environment update, not as an afterthought.
If you’re using a shared hosting environment and can’t control the PTR record, the most reliable long-term fix is moving critical sending to a dedicated outbound IP where reverse DNS can be set to match your SMTP banner.
If you’re seeing reverse DNS does not match SMTP banner alerts, run a reverse DNS lookup and an SMTP banner check. Then align the PTR record, the A record, and your email server’s identity so they match cleanly.
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Ready to tighten email security and protect deliverability? Book a demo to see how Sendmarc helps you implement and maintain SPF, DKIM, and DMARC, fix DNS issues that disrupt sending, and monitor for new risks as your environment changes. Secure your domains, safeguard your recipients, and build lasting brand trust.