What 550 5.7.28 Means
Gmail has rate-limited your sending IP. Your messages-per-time-window from this IP exceeded what Gmail tolerates for your IP's current reputation tier. Per Gmail's bulk-sender rules, the threshold depends on your reputation — higher reputation IPs get more headroom.
Gmail consumer accounts (@gmail.com) and Google Workspace domains. The 5.7.28 enhanced code is Gmail-specific.
You sent too many messages too fast for your reputation tier; you opened too many concurrent SMTP connections from one IP; or you sent in bursty patterns that look like volume-attack signatures. Common after large list imports where the sending platform doesn't throttle correctly.
How to Fix 550 5.7.28
- 1
Stop sending to Gmail immediately
Keep sending into the rate limit and the limit tightens further. Pause all Gmail-bound traffic for at least several hours and let the limit reset.
- 2
Check IP reputation in Postmaster Tools
postmaster.google.com shows your IP reputation tier. 'Bad' = severely rate-limited. 'Low' = moderately limited. 'Medium' = standard limits. 'High' = generous limits. Rate-limit recovery requires reputation improvement.
- 3
Cut your concurrent connection count
Many sending platforms open 5-20 concurrent SMTP connections per IP. Gmail interprets >2 concurrent connections from one IP as elevated rate. Configure your platform to cap at 1-2 connections per IP per destination. Most platforms have this setting under 'Sending Limits' or 'Throttle'.
- 4
Reduce daily send volume per mailbox
Even with low concurrency, total daily volume per mailbox matters. ColdRelay's recommended cap is 2 outbound emails/day per mailbox. Sending platforms that default to 30-50/day/mailbox routinely trip Gmail's rate limit. Drop to 2-5/day and 5.7.28 disappears.
Note: The 2/day cap is per-mailbox, not per-IP. With 100 mailboxes at 2/day, you can send 200/day to Gmail without rate-limit risk.
- 5
Spread your sends across the day
Sending 200 messages in a 5-minute window is worse than sending 200 messages across 8 hours. Configure your sending platform to spread sends across the work day (most platforms call this 'send window' or 'time spread').
- 6
Verify SPF, DKIM, DMARC pass
Rate limits are tighter for unauthenticated mail. Run the Email Deliverability Test at coldrelay.com/tools/email-deliverability-test. Fixing authentication often loosens rate limits within days.
References
550 5.7.28 in the Cold Email Context
Rate-limit errors are the structural risk of high-volume cold email. The math: Gmail will rate-limit any sender doing more than ~2-5 messages per minute from a single IP at 'Medium' reputation, and tighter at 'Low'. Shared-IP sending platforms hit rate limits constantly because many customers share the IP's volume budget. Dedicated IPs solve this: you get the full budget. ColdRelay's 2/day/mailbox volume cap is deliberately under Gmail's rate-limit threshold even at the lowest reputation tier — exceeding the cap is the primary cause of 5.7.28 in our customer base. Going to 100 mailboxes at 2/day = 200/day is structurally safer than going to 10 mailboxes at 20/day = the same volume but concentrated in fewer connections.
Frequently Asked Questions
What's Gmail's actual rate limit?
Gmail doesn't publish exact thresholds because they vary by IP reputation. Generally: 'High' reputation IPs can do ~30 messages/minute. 'Medium' ~10-15. 'Low' ~5. 'Bad' triggers immediate 5.7.28 on most sends. The numbers move based on volume history, engagement, and spam complaints.
How long does the rate limit last?
Short blocks: a few hours. Sustained blocks: days to weeks. Recovery is gradual — you can't 'unblock' instantly. Stop sending, let reputation rebuild, ramp volume slowly back up.
Is 5.7.28 the same as 4.7.28?
Close. 4.7.28 is the transient version (retry, will eventually deliver). 5.7.28 is permanent (give up for this send). Gmail uses both depending on severity — first hit might be 4.7.28; sustained hits escalate to 5.7.28.
Will switching to dedicated IPs fix this?
Yes, structurally. Dedicated IPs let you own your reputation budget instead of sharing it. ColdRelay's dedicated-per-customer IPs on isolated Azure tenants mean your IP's rate-limit ceiling is entirely yours. But you still have to send within Gmail's limits — dedicated IPs don't let you ignore rate limits, just guarantee you're not contaminated by others.
Do daily limits or rate limits matter more?
Both. Daily limits prevent accumulation; rate limits prevent bursts. A sender doing 1,000/day at 100/minute will trip rate limits even if 1,000/day is within their reputation tier's daily cap. Throttle both.