What is a PTR record?

A PTR record resolves an IP address to a domain name. It's the reverse of an A record and is crucial for email server authentication.

Will the email be delivered eventually?

If the underlying issue (like a missing PTR record) is not fixed, the retries will eventually fail and turn into a hard bounce.

← Error library

421 4.7.0 Temporary rate limit or block

temporary Rate Limit

Meaning

The receiving server has temporarily rejected the connection or message due to a policy issue, often related to rate limiting, low reputation, or missing authentication.

Common causes

The sending IP address does not have a valid PTR record (reverse DNS).
The sending IP or domain has a very low reputation due to spam complaints.
TLS is required for the recipient domain but was not used.

How to fix

Verify that your sending IP address has a valid PTR record that matches your forward DNS.
Ensure you are authenticating your emails with SPF, DKIM, and DMARC.
If sending to Gmail, review Google's Email Sender Guidelines and ensure TLS is enabled.

Provider notes

Gmail. Used for various temporary rejections, including missing PTR records, low IP/domain reputation, and TLS requirements.

Example bounce

421 4.7.0 The IP address sending this message does not have a PTR record, or the corresponding forward DNS entry does not point to the sending IP.

FAQ

What is a PTR record?: A PTR record resolves an IP address to a domain name. It's the reverse of an A record and is crucial for email server authentication.
Will the email be delivered eventually?: If the underlying issue (like a missing PTR record) is not fixed, the retries will eventually fail and turn into a hard bounce.