"550 5.7.1 Message rejected due to content restrictions" when you send email to external recipients in Exchange Online Protection
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Original KB number: 3024906
When an Office 365 or Exchange Online Protection user tries to send email to external recipients, the user receives the following non-delivery report (NDR):
550 5.7.1 Message rejected due to content restrictions
To resolve this issue, the sender or the associated administrator must contact either the recipient or the vendor of the recipient's anti-spam solution for additional analysis. In this situation, an anti-spam solution provider will typically update its signatures or provide a detailed explanation about why the email is being blocked.
This is not an issue that's caused by, or rooted in, Office 365 or Exchange Online Protection itself.
The sender has probably landed on a third-party anti-spam product's signature list. This may be caused by nonstandard email practices such as a failure to audit mailing lists. Recipients of these email messages have previously submitted those email messages as spam to their anti-spam product vendor. The vendor then added an identifier that's unique to the sender to its anti-spam definitions. It's this identifier that's now blocking the email.
Most frequently, the detection is triggered by something in the email signature. This might be a URL, domain name, or telephone number.
We recommend that you confirm the cause by sending a test email message to the same recipient after you completely remove the signature. If the email message is delivered without issue, you can confirm that a component or characteristic of the signature is causing the blocked email.
Still need help? Go to Microsoft Community.