SMTP reference

Gmail 4.7.28: what it means and how to fix it

Gmailrate_limitedaction: throttle

Gmail 4.7.28 is a rate limit, not a block. Gmail is telling you it has seen an unusual volume of mail from your IP or domain and is deferring rather than refusing it. The mail is still deliverable. Slow down and retry, and it will go through.

Worth being precise here, because it is the single most common misclassification in bounce handling. Gmail uses similar wording for two different outcomes: "unusual rate" with 4.7.28 is a deferral, while "likely unsolicited" with 5.7.1 is a block. Treating the deferral as a block means you stop sending when you only needed to slow down. Treating the block as a deferral means you keep retrying into a wall and make the reputation worse.

What it looks like in the log

421-4.7.28 Our system has detected an unusual rate of unsolicited mail originating from your IP address.

450-4.7.28 Gmail has detected an unusual rate of mail originating from your IP address.

Why it happens

Sending volume ramped faster than the IP's reputation supports, usually during warm-up.
A new IP or a range with no established sending history at Gmail.
A spike relative to your own baseline. Gmail reacts to the change more than the absolute number.
Complaint rate climbing, which tightens the rate Gmail is willing to accept.

What to do

Reduce concurrency and messages per connection for Gmail specifically, not globally.
Let the retry schedule work. This is a 4xx and the message is still queued.
Check Google Postmaster Tools for the same period. If the spam rate is climbing, the rate limit is a symptom and the complaint rate is the problem.
If you are mid warm-up, hold the current daily volume rather than continuing to ramp.

5.7.1 · 5.7.26

Classifying this automatically

This response maps to rate_limited with a recommended action of throttle in smtpsift, an open-source classifier for SMTP rejections and deferrals.

Gmail 4.7.28: what it means and how to fix it

What it looks like in the log

Why it happens

What to do

Related

Classifying this automatically