Received this failure response after emailing to an address I have successfully mailed to prior to moving to Office 365. I'm not sure if the address was rejected at the recipient's domain, or if Office 365 doesn't like the recipient's address and refused to send to it. Names and email addresses removed for privacy.
Root cause is usually the recipient in the case of 5.4.1. I would reach out to them after a retry in a few days (and you get a second bounceback) to get them (or their admins) to look into it - 5.4.1 can cover a few issues, it could be a misconfiguration on their end or it could be just a case of needing to whitelist your domain/address which should resolve the issue