Cpanel webmail 550 5.1.1 erro PLEASE HELP

Technical details of permanent failure: Google tried to deliver your message, but it was rejected by the server for the recipient domain domain.com by smtp.secureserver.net. The error that the other server returned was: 550 5.1.1 Recipient not found. Upper Details is the error i am facing Due to my customer complain that the e-mail they send to my Cpanel webmail has been bounce back and cannot been sent . i have find all kind of source :( still some required MX Entry to solve the problem but my Cpenal do not have MX Entry, is there anyway or anything that i can do at my sever to solve this problem :confused::confused::confused:::(:(:(:(:( If there is anybuddy can help me Thank You From : Mocha