How do I fix a rate limit?

You cannot bypass it manually. You must wait for the limit to lift, then resume sending at a much slower, controlled pace.

IP warmup is the practice of gradually increasing the volume of mail sent with a new IP address to build a positive sender reputation with ISPs.

← Error library

421 4.7.28 Unusual rate of email

temporary Rate Limit

Meaning

The receiving server (usually Gmail) has detected an unusually high volume of email from your IP or domain and has temporarily rate-limited your messages to protect users from spam.

Common causes

A sudden, massive spike in email volume from a new or cold IP address.
Sending a large burst of emails that looks suspicious or resembles a spam campaign.
High complaint rates triggering temporary throttling.

How to fix

Pause or slow down your email sending immediately.
Implement a proper IP and domain warmup strategy, gradually increasing volume over time.
Review your list hygiene to ensure you are only sending to engaged, opted-in users.

Provider notes

Gmail. Explicitly states 'Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited.'

Example bounce

421 4.7.28 Gmail has detected an unusual rate of email originating from your IP address. To protect our users from spam, email sent from your IP address has been temporarily rate limited.

FAQ

How do I fix a rate limit?: You cannot bypass it manually. You must wait for the limit to lift, then resume sending at a much slower, controlled pace.
What is IP warmup?: IP warmup is the practice of gradually increasing the volume of mail sent with a new IP address to build a positive sender reputation with ISPs.