Microsoft 365 5.7.513
Service unavailable, Client host [$ConnectingIP] blocked by $recipientDomain using Customer Block list (AS16012607)
Microsoft 365 5.7.513 means: service unavailable, Client host [$ConnectingIP] blocked by $recipientDomain using Customer Block list (AS16012607). It is a permanent failure, so do not retry it unchanged. Stop sending from the affected account or IP until the cause is fixed. Check for a compromised account, and remove unverified and unengaged addresses before you resume.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
Service unavailable, Client host [$ConnectingIP] blocked by $recipientDomain using Customer Block list (AS16012607)
Possible cause
The recipient domain added your sending IP address to its custom block list.
Provider's advice
The domain that received the email message blocked your sender's IP address. If you believe that your IP address was added to the recipient domain's custom block list in error, contact the recipient directly and ask them to remove it.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.7.513: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.7.0 other or undefined security status. |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | Partly |
In practice
Here the block is the recipient organisation's, not Microsoft's. The recipient's admin added your sending IP to their own block list in Microsoft 365. Microsoft cannot remove it. Contact the recipient through another channel and ask their administrator to remove the entry. If many senders share your IP, another sender may have caused the block. The block applies only to that one organisation, so mail to other Microsoft 365 tenants is not affected and nothing else needs to change.
What the sender should do
Stop sending from the affected account or IP until the cause is fixed. Check for a compromised account, and remove unverified and unengaged addresses before you resume.
Would verifying the list have prevented Microsoft 365 5.7.513?
Partly
The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
Common questions
- What does Microsoft 365 5.7.513 mean?
- Microsoft 365 5.7.513 means: service unavailable, Client host [$ConnectingIP] blocked by $recipientDomain using Customer Block list (AS16012607). 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 5.7.513?
- 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 5.7.513?
- 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