What 421 4.7.28 Means
Gmail's per-IP rate limit, transient version. Your sending IP has exceeded what Gmail will accept right now. Per Gmail's bulk-sender rules, this 4xx defer is meant to give the sender a chance to throttle before Gmail escalates to permanent 5.7.28 rejection.
Gmail consumer accounts and Google Workspace domains. 4.7.28 is the transient counterpart to 5.7.28; you usually see 4.7.28 first, and persistent over-sending escalates to 5.7.28.
Your IP sent more messages per time-window than Gmail's reputation tier for your IP allows. Bursty sending patterns, high concurrency from one IP, and pushing too many messages per mailbox all contribute.
How to Fix 421 4.7.28
- 1
Pause Gmail-bound sending for 1 hour
4.7.28 is the first warning. Continued sending into the throttle escalates to permanent 5.7.28. Pause for an hour minimum, then resume at lower rate.
- 2
Reduce per-mailbox daily send rate
Aim for 2-5 outbound messages per mailbox per day. ColdRelay's recommended cap is 2 outbound. Sending platforms that default to 30-50/mailbox routinely trip rate limits.
- 3
Lower concurrent connections
Cap concurrent SMTP connections from one IP to Gmail at 1-2 max. Most sending platforms have a 'Concurrency' or 'Connection Limit' setting. Reduce it under 'Sending Limits' or equivalent.
- 4
Check Postmaster Tools
postmaster.google.com shows your IP reputation tier. Bad/Low reputation means tighter rate limits. Medium/High gets more headroom. Fix reputation drivers (bounce rate, spam complaints) to lift your tier.
- 5
Spread sends over time
Bursty sending — 200 messages in 5 minutes then nothing — looks worse than smooth sending. Configure send-window to spread across the work day. Most platforms have 'send time spread' or 'jitter' options.
- 6
Scale by adding mailboxes, not pushing existing ones
Going from 50 to 100 mailboxes at 2/day = 200/day with no rate-limit risk. Going from 50 mailboxes at 2/day to 50 mailboxes at 4/day = same total but trips rate limits because per-mailbox volume crossed a reputation threshold. ColdRelay's per-mailbox pricing is designed for this scaling pattern.
References
421 4.7.28 in the Cold Email Context
4.7.28 is Gmail's polite warning that you're sending too fast. The escalation to 5.7.28 happens when you ignore the warning. Senders on shared IPs hit 4.7.28 constantly because they're competing with other tenants for the IP's rate budget. ColdRelay's dedicated-per-customer IPs eliminate cross-tenant rate-limit contention, and the 2/day/mailbox cap keeps you structurally under any reasonable rate-limit threshold. Combined: 4.7.28 events should be vanishingly rare for ColdRelay customers operating within recommended parameters.
Frequently Asked Questions
How fast does 4.7.28 escalate to 5.7.28?
Depends on persistence. A few isolated 4.7.28 events don't escalate. Sustained over-rate sending for hours typically escalates. Pause when you see 4.7.28; don't keep pushing.
Will retrying help?
Yes — 4xx is transient. Wait an hour and the next batch typically goes through. But if you're chronically over-rate, retries just delay the inevitable 5.7.28 escalation.
Does 4.7.28 affect reputation?
Marginally. Receivers track 4xx-to-2xx conversion. Most 4.7.28 events eventually convert successfully on retry, which is reputation-neutral. Sustained 4.7.28 without resolution does slowly degrade reputation.
Why do other receivers not have a 4.7.28 equivalent?
They do — just with different enhanced codes. Microsoft uses 4.7.500. Yahoo uses 4.7.0 variants. The concept (transient rate limit) is universal; the code is provider-specific.