RESOLVED:
Our engineers have implemented a fix to resolve the issue with email notifications, users should no longer expect to receive quota notifications.
We are currently investigating emails being sent in error to users by our mail service:
"Your mailbox is now at%. If this reaches 100%, you will not be able to receive any more mail. Please clear your mailbox so that you can continue to receive new messages."
There is no percentage specified in these notifications, and they have no impact on the use of the mailboxes, sending, and receiving. We are currently investigating if there is anything in common between the email accounts receiving these notices.
The users can disregard the emails and continue to use their accounts as normal. We will post updates as they are available. Sorry for the inconvenience that this has caused you.