Temporary — will retry
SMTP 421 4.7.26
rate limited, unauthenticated (Gmail)
SMTP 421 4.7.26 is Gmail rate limiting mail because it is not authenticated. Google's message reads "This email has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM." It is temporary, but it becomes a permanent 550 5.7.26 if nothing changes.
How it appears
421 4.7.26 This email has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM.
What causes it
- The message passed neither SPF nor DKIM.
- A new sending service was added without updating SPF or setting up DKIM signing.
- The SPF record is broken, for example by going over the 10-lookup limit.
How to fix it
In order — try the first one first.
- Set up DKIM signing for the domain you send from.
- Make sure every service that sends as your domain is in your SPF record.
- Publish a DMARC record. Google requires it for bulk senders.
- Audit your domain to confirm SPF, DKIM and DMARC all pass.
Common questions
- What does SMTP 421 4.7.26 mean?
- SMTP 421 4.7.26 is Gmail rate limiting mail because it is not authenticated. Google's message reads "This email has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM." It is temporary, but it becomes a permanent 550 5.7.26 if nothing changes.
- Should I retry a 421 4.7.26 error?
- Yes, and your sending server does it automatically. 421 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.
- How do I fix SMTP 421 4.7.26?
- Set up DKIM signing for the domain you send from.
Other 421 replies
Last reviewed 2026-09-11