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Error: "Recipient address rejected: 5.2.2 Over quota 554 5.0.0 Service unavailable" ?
This error message means that your correspondent's mailbox has a limited storage capacity and that this limit has currently been reached, or else that the message you are trying to send him is too big for his allocated space (or the remaining space in his mailbox).
You need to contact your correspondent and ask him to empty his mailbox or ask what the maximum message size is for any email you send him.