Attached is a screenshot of what I see for a specific user.
She is complaining of not receiving emails anymore so I guess it's true.
But I am having a hard time understanding, how can she be "over" quota if both numbers are below 100%?
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Could it be that she uses Gmail as a collect address form another email?
If that is the case it could be that her other email box is full and emails are not delivered to that address resulting to only those emails not delivered to Gmail (or anywhere else for that matter).
Links with possible solutions to your issue
Keep in mind that the Storage on Legacy edition is 15 GB for each user on your Free Edition