I was using an email to send bulk emails. now the email is blocked. As an admin, how can I unblock it.

If the mailbox was blocked in Exchange Online for suspected spam or exceeding outbound sending limits, unblock it in the Microsoft Defender portal:

1. Go to Microsoft Defender portal.
2. Open Email & collaboration > Review > Restricted entities.
3. Find the blocked account. The Entity value is Mailbox.
4. Select the mailbox, then select Unblock.
5. In the Unblock user flyout, review the Overview and Recommendations to confirm the account is not compromised.
6. On the Unblock user page, use the links there to Enable MFA and Reset the user's password if not already done.
7. Select Submit, then confirm with Yes.

Expected result: under normal circumstances, sending restrictions are removed within one hour, and no longer than 24 hours in transient cases.

Before unblocking, verify why the block happened:

• Check whether the account was compromised by reviewing sign-in activity, unknown inbox rules or forwarding rules, and recent sent items.
• If the mailbox was used for legitimate bulk sending, note that Exchange Online isn't designed for bulk mailing scenarios.

For future bulk email, use a service designed for high-volume sending, such as Azure Communication Services Email. If application or device mail must go through Microsoft 365, use supported methods like SMTP relay via Microsoft 365 or Direct Send. Also configure SPF, DKIM, and DMARC for the sending domain.

References:

• Remove blocked users from the Restricted entities page
• Troubleshoot outbound sending limits and blocked users in Exchange Online