Users of Cox Business Email may encounter a returned message error when attempting to send an email. A 552 email error usually occurs when an attachment in the message can't be read. Emails submitted with message sizes that are larger than any of the allowed limits for an email user account will be rejected with an error of 552 messages. In order to resolve Cox email error codes, such as a valid email bounced with error 552, the attachment limit needs to be increased on the MTA (Mail Transfer Agent) or Transport Hub.
see also
Cox Email Sign In Issues, Why is My Cox email not working Today