We have about a dozen people with mail accounts under Office 365. Today whenever any of use send email to a specific person (within our company) they receive the e-mail, but the sender receives back the message below:
The recipient's mailbox is full and can't accept messages now. Please try resending your message later, or contact the recipient directly.
It only seems to happen when the sender is within our company, not when sent from an external mail account.
I have checked the inbox rules on his account and there are no rules that would forward the e-mail to this external account. I don't see any mail mail flow rules either. How is this happening, how do I turn it off, and how did it likely get set up in the first place (I know that will likely just be that an account was hacked).