Office 365 will automatically block a user account if they are sending too many emails classified as spam.
This article shows you how to unblock a user in Office 365 using the Security & Compliance portal.
1. A user will get a message like "Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send messages outside of your organization. Contact your email admin for assistance."
It is advisable to change the users email password and check their computer for malware or viruses. A full sultion can be found https://support.microsoft.com/en-gb/help/2551603/how-to-determine-whether-your-office-365-account-has-been-compromised
2. Once you have reset the users password and are confident that the account in no longer compromised you will need to unblock the user. Login to office.com and got the the Security & Compliance option. If you don't see this then you need to get your Office 365 administrator to do this.
3. Click on Alerts --> Dashboard and then scroll down to Restricted Users which will take you to https://protection.office.com/#/restrictedusers
4. Click 'Unblock'