@Mail Domain Check

Gmail ยท SMTP 451 / 421 4.7.23

Gmail 451 4.7.23: fix the sending IP PTR rate limit

Gmail temporarily limits mail when the actual connecting IP has no PTR record or its PTR hostname does not resolve forward to that IP. Check the exact bounce IP below.

What 4.7.23 means

Google's current SMTP error table lists 451 4.7.23 as a temporary rate limit caused by missing reverse DNS or a forward-DNS mismatch on the sending IP. Older SMTP transcripts and help threads can show 421 4.7.23 with the same underlying requirement.

A 4xx response tells the sending queue to retry later; it does not prove the next delivery will succeed. Correct the DNS fault before increasing retries or resending a campaign.

Test the IP named in the bounce

  1. Copy the IPv4 or IPv6 address that connected to Gmail. Do not substitute your website IP, MX host, or visible From domain.
  2. Query that address for PTR. The result should be a stable hostname controlled by your mail or network provider.
  3. Query the PTR hostname for A when the sender uses IPv4, or AAAA for IPv6.
  4. At least one forward result must equal the original sending IP. Merely having a PTR record is not forward-confirmed reverse DNS.

The scanner above performs these reverse and forward queries and compares IP addresses byte-for-byte, including compressed IPv6 forms.

Fix the correct DNS owner

PTR lives in the reverse-DNS zone delegated to the IP address owner. A hosting company, data center, ISP, or email service usually controls it. The DNS operator for the returned hostname controls its forward A or AAAA record. Updating ordinary domain DNS alone will not repair a reverse zone owned by another provider.

For a sending pool, repeat the check for every outbound IP. One passing node does not prove that Gmail will see the same DNS from another pool member.

Retry without creating another rate spike

  1. Wait until the authoritative PTR and forward records are visible through public resolvers.
  2. Send one new test through the same outbound IP and inspect the new SMTP response.
  3. Keep normal exponential queue backoff. Do not force a bulk retry while 4.7.23 is still returned.
  4. If the response changes to 550 5.7.25, treat it as the permanent form of the same PTR or forward-DNS failure.

What this check can prove

The tool can verify public PTR and forward A/AAAA consistency at scan time. It cannot observe which IP a later message will use, Gmail's private reputation signals, your queue policy, or whether every member of a shared sending pool is healthy. A passing result removes one documented cause; it does not guarantee inbox placement.

Primary sources