Views:
Microsoft maintains a blacklist of accounts and domains suspected of sending spam and emails sent to exchange accounts are compared against this and can be blocked or flagged if they are found to be on it.

One of the reasons this can happen is if a normal exchange account attempts to send thirty or more emails in one go.
 
To remove an address from the black list, visit this url and follow the steps its asks for. Once the request has been submitted it can take multiple days for it to have an effect.
Keywords: Outlook, Exchange, Email, Office, Office 365, Spam