iCloud email bounce message: "user is overquota"
What does user is over quota mean in reference to Apple's email software?


The user's storage, including the mailbox, is full.

Each iCloud account is allocated a certain amount of space, much like a drive on your computer.  Once that space is filled up, there's no more room to accept any more, including email.

The solution to this is for the user to "clean up" their mailbox and/or files they've stored on their account.  Most of the time, this is caused by someone never deleting email.  It can be remedied by deleting old obsolete mail.

Unfortunately, you as the sender can't do anything, but contact the recipient by other means (phone call, text message, etc.) and let them know of the issue.