Permanent failure — do not retry
SMTP 550 5.7.28
unusual rate of unsolicited mail (Gmail)
SMTP 550 5.7.28 means Gmail is blocking mail from your IP because it has seen an unusual rate of unsolicited email from it. Google's message reads "There is an unusual rate of unsolicited email originating from your IP address." It is the hard version of the 421 4.7.28 deferral.
How it appears
550 5.7.28 There is an unusual rate of unsolicited email originating from your IP address. To protect our users from spam, mail sent from your IP address has been blocked.
What causes it
- A burst of cold or bulk email from an IP with little history.
- High spam complaint rates or spam-trap hits.
- A compromised server or account sending spam from your IP.
How to fix it
In order — try the first one first.
- Stop sending to Gmail from that IP until you have found the cause.
- Check Google Postmaster Tools for spam rate and IP reputation.
- Only send to people who expect your mail, and clean the list before resuming.
- Resume at low volume and increase it gradually.
Common questions
- What does SMTP 550 5.7.28 mean?
- SMTP 550 5.7.28 means Gmail is blocking mail from your IP because it has seen an unusual rate of unsolicited email from it. Google's message reads "There is an unusual rate of unsolicited email originating from your IP address." It is the hard version of the 421 4.7.28 deferral.
- Should I retry a 550 5.7.28 error?
- No. 550 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 550 5.7.28?
- Stop sending to Gmail from that IP until you have found the cause.
Other 550 replies
- 550 5.1.1 — mailbox does not exist
- 550 5.7.1 — message refused by policy
- 550 5.4.1 — recipient address rejected, access denied
- 550 5.2.1 — mailbox disabled, not accepting messages
- 550 — mailbox unavailable
- 550 5.1.0 — address rejected, other address status
- 550 5.1.10 — recipient not found (Microsoft 365)
- 550 5.1.8 — bad outbound sender (Microsoft 365)
- 550 5.4.5 — daily sending limit exceeded (Gmail)
- 550 4.4.7 — message expired in the queue
- 550 5.7.25 — no PTR record for sending IP (Gmail)
- 550 5.7.26 — blocked, sender unauthenticated (Gmail)
- 550 5.7.27 — SPF authentication failed (Gmail)
- 550 5.7.23 — SPF violation (Microsoft 365)
- 550 5.7.124 — sender not in allowed senders list (Microsoft 365)
- 550 5.7.509 — DMARC failed, policy reject (Microsoft 365)
- 550 5.7.606-649 — banned sending IP (Microsoft)
- 550 5.7.705 — tenant has exceeded threshold (Microsoft 365)
- 550 5.7.708 — traffic not accepted from this IP (Microsoft 365)
- 550 5.7.750 — blocked from sending from unregistered domains (Microsoft 365)
- 550 5.7.515 — authentication required for high-volume senders (Outlook.com)
- 550 5.7.1 S3140 — IP range on Outlook.com block list
Last reviewed 2026-09-11