What Does Gmail's 421 4.7.28 Error Mean, and How Do I Fix It?

What it means

421 is a temporary failure: Gmail didn't accept the message now, but your server can retry later. 4.7.28 is Gmail's rate limit for unusual or unsolicited volume. Google's SMTP error reference lists several texts for it. The part after the code tells you which quota you hit:

Google also has a 550 5.7.28 variant, a permanent block for "an unusual rate of unsolicited email" from your IP address. That one needs more than waiting. If the code is 421 4.7.0 instead, the cause is reputation, PTR or content; see 421 4.7.0. The email bounce decoder tells the variants apart from the pasted bounce.

Why it happens with cold email

Gmail tracks volume and feedback per domain and per IP. Google's sender guidelines explain that DKIM and SPF quotas are specific to your domain, while "The IP address quota is shared for all senders that use that IP address. When the IP address hits its quota, all domains that send from that IP address stop sending emails."

Common causes:

• A jump in volume, or sending in bursts.

• Too many mailboxes on one domain, which adds up to a high rate for that domain.

• Other senders on the same shared IP or IP block.

• The same link domain in many emails at once.

A case from our own infrastructure, July 2026

Per-mailbox volume barely changed. The jump came from fewer IPs carrying the same mail, so each IP's Google traffic rose by half in one day and had doubled a day later. One affected /24 range took about two months to get back to normal inbox placement.

Two months later we saw the opposite pattern. One sending IP delivered about 185,000 emails to Google in a week with 28 deferrals, and inbox placement still dropped: Google moved the mail to spam without slowing it down. Google can filter silently, so a log with few 421s does not prove your mail lands in the inbox.

What Google says to do

From the sender guidelines, when you get 4.7.28:

1. "Do not send email for at least 10 minutes."

2. "Identify the cause."

3. "If the error message does not specify if you exceeded your DKIM, SPF, or IP quota, assume that all three are affected."

4. "After 10 minutes, send emails from a single connection."

5. "If the single connection fails, wait another 10 minutes."

6. "If successful, increase the number of connections one at a time until your desired number of connections are established."

The Postmaster Tools delivery error "Rate limit exceeded" gives the same advice: stop, resume in a few minutes, and "Send email at a constant rate."

What to change so it doesn't come back

1. Spread sends across the day. Use sending windows and gaps between emails.

2. Stay within per-mailbox and per-domain limits. See cold emails per mailbox per day and mailboxes per domain.

3. Remove links from the first email if the text names a URL domain.

4. Raise volume in steps. Google: "Avoid introducing sudden volume spikes if you do not have a history of sending large volumes."

5. If the text names the IP or netblock and your own volume is modest, the IPs are shared with heavier senders. Ask your mailbox provider.

6. Watch volume per sending IP as well as per mailbox. In our case the deferrals started the day Google-bound mail per IP went from about 2,100 to 3,100, with per-mailbox volume flat. On shared or provider IPs, ask how much Google mail each IP carries a day and whether IPs are being consolidated.

7. Treat a burst of 421s as a stop signal. Placement kept falling the day after the spike, and the range took about two months to recover. Pause and cut volume on the first day.

8. Check placement even without 421s. Run a placement test with Gmail and Google Workspace seeds and watch reply rates, because Google can send mail to spam with almost no deferrals.

What we recommend at Outreach2day

Up to 25 cold emails per mailbox a day, excluding warmup, and up to 5 mailboxes per domain. At those levels a domain sends at most 125 cold emails a day, which keeps per-domain volume low.