If you have a question in mind “Why is my account blocked?” then we have solved your problem.
One way an account can get blocked is due to exceeding the email outbound limit set by Microsoft. There are some limitations set by Office 365:
If any of the above criteria are met, the account is blocked and then their emails will be directly flowing to the Junk/Spam mailbox on the recipient’s end.
We can also use Exchange Online PowerShell to View and remove users from the Restricted user list.
Get-BlockedSenderAddress -SenderAddress <emailaddress>
Remove-BlockedSenderAddress -SenderAddress <emailaddress>
Note: Last thing about the Restricted user: OWA will create an automatic rule when the user was blocked.
So you will need to login into user’s OWA, find the rule and delete it.
Basically the rule will move all the incoming emails to Junk folder, and this rule can only be found in OWA and not in Outlook application.