421 4.7.28: Unusual sending rate or unsolicited mail: rate limited

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). The IANA registry 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, 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 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.
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: Temporarily refused: reputation, DNS or TLS
• 450 4.2.1: The recipient is receiving mail too fast: try later
• 550 5.7.1: Delivery not authorized: refused for policy reasons
• 550 5.4.5: The sender reached a daily sending limit (Google)

See every bounce code in the dictionary.

Many bounces from addresses that do not exist can be avoided: check an address with our free email checker, or clean a whole list before you send.

How we wrote this: drafted by the email-validation.org team with the help of AI writing tools, from our product documentation and the sources linked in the text.