# 421 4.7.28 Bounce: Unusual Rate of Unsolicited Mail

URL: https://email-validation.org/bounce-codes/421-4-7-28/
Updated: 2026-11-06

> Documentation index: https://email-validation.org/llms.txt

**At a glance**

- **Type:** temporary failure (class 4, RFC 3463 §2). The sending server keeps the message and retries later (RFC 5321 §4.5.4.1); it bounces only if the problem lasts until the message expires.
- **Basic code 421:** “Service not available, closing transmission channel” (RFC 5321 §4.2.3).
- **Enhanced code 4.7.28:** “Mail flood detected” in the IANA registry (draft-levine-mailbomb-header), in the group security or policy status (RFC 3463 §3.8).

A 421 4.7.28 reply means Gmail noticed an unusual rate of unsolicited mail tied to you and is slowing you down. Gmail can also block such mail outright, with 550 5.7.28.

## What it means

The 421 code is a temporary refusal that closes the connection; your server should retry later ([RFC 5321 §4.2.1](https://www.rfc-editor.org/rfc/rfc5321)). The [IANA registry](https://www.iana.org/assignments/smtp-enhanced-status-codes/) lists X.7.28 as "mail flood detected", from the mailbomb header draft. Gmail's use is close to that name: too much mail of a kind its users do not want, arriving too fast.

## How providers use it

According to [Google's SMTP error reference](https://support.google.com/a/answer/3726730), Gmail sends 421 4.7.28 when it sees an unusual rate of unsolicited mail coming from any of these:

- your sending IP address, or the network block it sits in;
- your DKIM signing domain;
- your SPF domain;
- a domain used in the links in your messages.

For the same pattern from an IP, Gmail can also refuse outright with 550 5.7.28. Note the last item: a link domain shared with other senders, such as a public link shortener, can count against your mail even when your own IP and domain are clean.

## Common causes

- A sudden jump in volume, such as a first send to a large list or a new IP that has no history.
- A list with old, bought or scraped addresses, which brings complaints and bounces.
- Messages people did not expect, or with no easy way to unsubscribe, so they mark them as spam.
- A compromised account or form sending spam through your systems.

## How to fix it

1. Slow down. Let your server's queue retry at a lower rate; RFC 5321 §4.5.4.1 suggests at least 30 minutes between retries.
2. Check your spam rate in Postmaster Tools. [Google's sender guidelines](https://support.google.com/a/answer/81126) ask senders to keep it low.
3. Send only to people who asked for your mail. Remove addresses that hard bounce or never engage; see [email list cleaning](https://email-validation.org/guides/email-list-cleaning/).
4. Bulk senders need SPF, DKIM and DMARC, and one-click unsubscribe on marketing and subscribed messages, under the same guidelines.
5. Look at every link domain in the message, and drop any shared shortener.
6. Rule out a compromised account or an abused signup form before you resume.

## What to do with the address

Keep it. A 4.7.28 deferral is about your sending pattern, not this mailbox. Do review how the address joined your list.

## Related codes

- [421 4.7.0](https://email-validation.org/bounce-codes/421-4-7-0/): Temporarily refused: reputation, DNS or TLS
- [450 4.2.1](https://email-validation.org/bounce-codes/450-4-2-1/): The recipient is receiving mail too fast: try later
- [550 5.7.1](https://email-validation.org/bounce-codes/550-5-7-1/): Delivery not authorized: refused for policy reasons
- [550 5.4.5](https://email-validation.org/bounce-codes/550-5-4-5/): The sender reached a daily sending limit (Google)

See [every bounce code](https://email-validation.org/bounce-codes/) in the dictionary.

Many bounces from addresses that do not exist can be avoided: check an address with our [free email checker](https://email-validation.org/email-checker/), or [clean a whole list](https://email-validation.org/bulk-email-verification/) before you send.