O365 Exchange – user email marked as spam cannot send email outside organization

According to the error message, this issue may be caused by your SMTP address or IP address is blocked by the EOP. So, please let the admin go to the Exchange Admin Center (EAC) to check if your SMTP address/IP address is blocking and delist it:

1. In the EAC, navigate to protection > action center.

2. Select the Search icon and enter the SMTP address of the blocked user.

3. Click Unblock Account in the description pane.

4. Click Yes to confirm the change.

 

https://answers.microsoft.com/en-us/msoffice/forum/all/cant-send-emails-outside-organization/a70914b3-ee9c-411d-9d3d-7e5db9c75a91?auth=1

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