smtp;452 4.2.2 The email account that you tried to reach is over quota. Please direct the recipient to https://support.google.com/mail/?p=OverQuotaTemp - gsmtp
Google Mail has has restrictions on the amount of storage a user can use, the recepiepnt is over their storage limit and unable to receive new messages. They will need to delete emails to start to receive messages again. Once they've done this, they'll be able to start to receive messages again.