Microsoft 365 5.7.511
Access denied, banned sender
Microsoft 365 5.7.511 means: access denied, banned sender. It is a permanent failure, so do not retry it unchanged. Stop sending from the affected account or IP until the cause is fixed. Check for a compromised account, and remove unverified and unengaged addresses before you resume.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
Access denied, banned sender
Possible cause
The IP that you're trying to send from was banned.
Provider's advice
To delist the address, email [email protected] and provide the full NDR code and IP address.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.7.511: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.7.0 other or undefined security status. |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | Partly |
In practice
Microsoft has banned the sending IP. To get it removed, Microsoft asks you to email [email protected] with the full NDR and the IP address. Before asking, fix whatever caused the listing, since a repeat listing is harder to clear. On shared IPs, another sender's mail can cause the ban, which is a reason to use a sending service with good IP hygiene. Microsoft's text also says this is a banned IP, not a banned account, so changing the sending account will not help. Check the IP against major blocklists, look for compromised accounts or open relays on your server, and review recent sending volume. Then request delisting with the full NDR. Keep the NDR text exactly as received.
What the sender should do
Stop sending from the affected account or IP until the cause is fixed. Check for a compromised account, and remove unverified and unengaged addresses before you resume.
Would verifying the list have prevented Microsoft 365 5.7.511?
Partly
The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
Common questions
- What does Microsoft 365 5.7.511 mean?
- Microsoft 365 5.7.511 means: access denied, banned sender. It is a permanent failure, so do not retry it unchanged. Stop sending from the affected account or IP until the cause is fixed. Check for a compromised account, and remove unverified and unengaged addresses before you resume.
- Should I retry after Microsoft 365 5.7.511?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. Stop sending from the affected account or IP until the cause is fixed. Check for a compromised account, and remove unverified and unengaged addresses before you resume.
- Would verifying my list have prevented Microsoft 365 5.7.511?
- Partly. The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
Sources
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05