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.

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 instead of spread over the day.

• 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.

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 instead of batches.

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.

What we recommend at Outreach2day

Up to 25 cold emails per mailbox a day, not counting warm-up, 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.