Remove blocked users from the Restricted Users portal in Office 365

In Microsoft 365 organizations with mailboxes in Exchange Online or standalone Exchange Online Protection (EOP) organizations without Exchange Online mailboxes, outbound email messages that are sent through EOP are automatically checked for spam and unusual sending activity.

Admins can remove users from the Restricted Senders portal in the Security & Compliance Center or in Exchange Online PowerShell as mentioned below,

In the Security & Compliance Center, go to Threat management > Review > Restricted users.

1. Find and select the user that you want to unblock. In the Actions column, click Unblock.

2. A fly-out will go into the details about the account whose sending is restricted. You should go through the recommendations to ensure you're taking the proper actions in case the account is actually compromised. Click Next when done.

3. The next screen has recommendations to help prevent future compromise. Enabling multi-factor authentication (MFA) and changing the passwords are a good defense. Click Unblock user when done.

4. Click Yes to confirm the change.

    Note: It might take up to 24 hours for all restrictions to be removed from the user.

Use Exchange Online PowerShell to view and remove users from the Restricted Users list,

View this list of users that are restricted from sending email, run the following command:

Get-BlockedSenderAddress

To remove a user from the Restricted Users list, replace <emailaddress> with their email address and run the following command:

Remove-BlockedSenderAddress -SenderAddress <emailaddress>