Your registration to the Support center does not link to your hosting account with us, so I'm not sure which account is being affected exactly.
However, whenever we see the error of "550-Mailbox quota exceeded", it is normally related to the email account being full. You simply need to login to your Cpanel, go to EMAIL, and change the quota so that it's larger, if you want the simple way out of the problem. Exceeded the quota means that the mailbox is full and you can no longer send or receive emails from that address.
Here's an article about this issue that can give you more info and the steps to fix if needed: 550-Mailbox quota Exceeded - How to fix in Cpanel
The other way to fix it would be to login to the email account affected by the quota and remove emails from it until it no longer violates the quota. However, since you're on a shared server where the limits can be quite large, it may be simpler to make the quota larger to avoid this problem in the future.
If you have any further questions, please contact technical support or leave a comment at the bottom of the page.
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