TL;DR: Throttle the send rate and spread the campaign over more time - 421 4.7.0 is Gmail telling the agent it is sending too fast, and the fix is pacing, not pushing. Drop to a modest per-minute rate, queue the remainder, and let the run take the hours it needs. If the volume is real and recurring, move bulk sending to a proper email service.

```text
421 4.7.0 "too many messages"
```

1. Pause sending immediately when the 421s start; continuing to push extends the throttle. Expected: the send queue stops growing the problem.
2. Set a send rate well under Gmail's limits: a small number of messages per minute with pauses between them, applied per sending account. Expected: the throttle clears and sends start being accepted again.
3. Queue the remaining messages and drip them out at the throttled pace instead of all at once. Expected: the backlog drains steadily with no new 421s.
4. Split the load if you run multiple campaigns: stagger their schedules so they never burst together from the same account. Expected: combined send rate stays under the limit at all times.
5. If the campaign volume keeps hitting the ceiling, move bulk outreach to a dedicated sending service with its own warmed-up domain and throttle the agent's Gmail use to human-scale mail. Expected: bulk volume leaves Gmail entirely.

## Use this when
- Gmail answers with 421 4.7.0 too many messages during an outreach run
- The agent sends in bursts from a Gmail or Workspace account
- Throttles recur on every campaign

## Not for this skill when
- The error is 550 5.1.1 (dead addresses - validate the list instead)
- The error is 5.7.1 content-based (the message trips a filter - different fix)
- The account is suspended (that needs human review and appeal, not pacing)

## Variant phrasings
- Gmail SMTP too many messages throttle
- 421 4.7.0 Gmail rate limit outreach
- Gmail blocking bulk sends from agent

## Why it happens
Gmail caps how many messages an account can send per day and how fast it can send them; the caps are lower for new or low-reputation accounts. An outreach agent firing a whole campaign in one burst trips the rate cap in minutes. Gmail's 421 is a slow down signal, and ignoring it turns a short throttle into a longer one.

## Edge cases
- The throttle is per account: spreading the same burst across several Gmail accounts to dodge it is evasion and risks all of them; fix the pace instead.
- Workspace accounts have higher limits than consumer Gmail but they still have limits; do not assume a Workspace account is uncapped.
- A 421 storm can precede a temporary sending suspension; if Gmail starts deferring everything, stop entirely and wait a full day before resuming at a slower pace.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_9MfCNAWAflGsCXw-7WWYUQ