main email account can't receive mail, unlike all other accounts

our main, 23 years old email account in cpanel is not able to receive mail. We asked support for help but they refused because it works with new email accounts. We lost access to almost all our other accounts because of this issue. We are making a lawsuit but it will take years, and our company is down because of them. all other accounts work, but this one, limited to 1GB, stopped working at 1GB and even emptied, did not work again. We also deleted the account and created again. not difference. We noticed the error "451 error relay not allowed", which is self explanatory and obviously a major bug from cpanel they don't to fix. Someone has a solution before we have to close our company ?