our microsoft business mailbox was used to send unauthorised phishing email

Hi Cmacattack,

If you have a compromised Microsoft 365 email account, please ask your IT admin to use the steps in Secure and Restore Email Function to a Compromised Microsoft 365 Mail Enabled Account to regain control of the account.

However, if you're an IT admin, you should investigate the compromised account before restore their access to your user.

If you disabled the account during the investigation, reset the password and then enable the account

If the account was used to send spam or a high volume of email, it's likely that the mailbox is blocked from sending mail.

For more information, please consult Respond to a compromised email account in Microsoft 365 - Microsoft Defender for Office 365 | Micro…

I hope this provides enough clarity and helps you stay inform with your current situation.

Thank you for your patience and your understanding. If you have any questions or need further assistance, please feel free to share them in the comments on this post so I can continue to support you.  

If the answer is helpful, please click 'Yes' and kindly upvote it.

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