What 421 4.7.26 means
The message Gmail attaches to this code is “This email has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM.”, as written on its reference page. The first digit says how final it is: a 4xx reply is temporary, so the sending server queues the message and retries. The enhanced status 4.7.26 says what class of problem it is: authentication. The receiver could not authenticate the message: SPF, DKIM or DMARC failed or is missing. A DNS fix, not a reputation fix.
Why it happens
The receiving server checked who sent the message and could not make the answer add up. Either the sending service is not listed in the domain's SPF record, the DKIM signature is missing or does not verify, or the domain that passed authentication is not the domain in the From header, which is what alignment means. Nothing about volume or reputation changes this: the record is either right or it is not.
What to do first: Temporary form of 5.7.26, same fix: authenticate with SPF or DKIM and keep the volume steady while the records propagate.
Why warming does not fix this
Warming builds sender reputation. 421 4.7.26 in this form is a authentication problem, and no amount of engagement history changes it. Fix the cause and the bounce stops; warming is for what comes after, when the mail is accepted and the question becomes which folder it lands in. See how warming works for that part.
How to fix it, in order
- Run the auth checker on the sending domain
- Publish or repair the failing record (SPF, DKIM or DMARC)
- Check alignment with a message's headers in the auth checker
Related codes
- 550 5.7.515 (Microsoft): authentication level not met
- 550 5.7.509 (Microsoft): DMARC verification failed
- 550 5.7.23 (Microsoft): SPF violation
- 550 5.7.27 (Gmail): SPF authentication failed
- 550 5.7.30 (Gmail): DKIM authentication failed
More on Gmail
- Google Workspace emails going to spam: the Postmaster-led diagnosis, dashboard by dashboard
- Gmail and Yahoo bulk sender requirements: the rules behind Google's authentication and spam-rate rejections
- Google Postmaster Tools domain reputation: what the reputation tiers and the spam rate mean
Questions about 421 4.7.26
What does 421 4.7.26 mean?
Gmail returns 421 4.7.26 with the message "This email has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM.": rate limited, unauthenticated. The receiver could not authenticate the message: SPF, DKIM or DMARC failed or is missing. A DNS fix, not a reputation fix.
Is 421 4.7.26 a hard bounce or a soft bounce?
A soft bounce: the 421 reply is temporary and the sending server retries. If it persists for days, treat it as the permanent form and fix the cause.
How do I fix 421 4.7.26?
Temporary form of 5.7.26, same fix: authenticate with SPF or DKIM and keep the volume steady while the records propagate. Gmail's own reference for the code is linked on this page.
Does email warmup fix 421 4.7.26?
No. 421 4.7.26 in this form is a authentication problem, and warming builds reputation, which is not what is failing here. Fix the cause and the bounce stops.