Exchange Status Code 510

I have a client who has been using Office 365 for a few a week now with no real problems, all of a sudden today they have started to have mails returned with the error below

Diagnostic code = OrNameUnrecognized; Reason code = TransferFailed; Status code = 510

            < #5.1.0 smtp;550 5.1.0 RESOLVER.ADR.InvalidInSmtp; encapsulated INVALID address inside an SMTP address (IMCEAINVALID-)>

This appears to be a little random some of the addresses are in the address book whilst others aren't, they have sent succesfully to some of these adresses last week and others are first time

Regards

Barry