<If user is receiving this bounce back message>

"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."

If receiving this there needs to be some changes to the account in case the account was compromised: Reset the user password, consider enabling two factor authentication on that mailbox.

 Once the account is verified as remediated or properly secured you will need to check to see if the account has become restricted and no longer able to send emails.

1. Log onto the Microsoft Security and Compliance portal: www.security.microsoft.com
   1. Log on with the O365 administrator credentials for the email domain
2. At the home portal click under Email and Collaboration menu click “Review” 

3.After clicking Review then select “Restricted Users”

4. Restricted users will be listed:

5. Click the user that needs to be unblocked and select “unblock”

6. The entry should go out of the listing when it returns to the above screen