Rackspace Create Mailbox Error: Entity of type Mailbox identified by [email protected] was not found

Question

I am trying to hit API via postman to create a mailbox Rackspace URL: http://api.emailsrvr.com/v1/customers/123456/domains/example.com/rs/mailboxes/newuser

Body: { 
  "password" : "Secret13!@#",
  "size" : "2048",
  "enableVacationMessage" : "true",
  "vacationMessage" : "My Vacation Message",
  "emailForwardingAddresses" : "[email protected]",
  "saveForwardedEmail" : "false",
  "lastName" : "Testlastname",
  "firstName" :  "Jay",
  "generationQualifier" :  "III",
  "initials" :  "A",
  "organizationUnit" :  "IT",
  "businessStreet" :  "123 Biz St.",
  "businessCity" :  "Blacksburg",
  "businessState" :  "VA",
  "businessPostalCode" :  "24060",
  "businessCountry" :  "USA",
  "homeStreet" :  "456 Home St.",
  "homeCity" :  "Charlotte",
  "homeState" :  "NC",
  "homePostalCode" :  "28210",
  "homeCountry" :  "USA",
  "notes" :  "This is my note about my mailbox.",
  "title" :  "My Title",
  "userID" :  "123456",
  "customID" : "123456",
  "organizationalStatus" :  "OrgStatus",
  "employeeType" :  "Full-Time",
  "visibleInExchangeGAL":"true", 
  "visibleInRackspaceEmailCompanyDirectory" : "false"
  
}

But it is giving me error:

Entity of type Mailbox identified by [email protected] was not found

I have checked the Rackspace documentation and using the exact syntax mentioned there.