Message could not be sent because it is too large (552 - 552 <campmaster@cox.net> message rejected. Refer to Error Codes section at https://www.cox.com/residential/support/email-error-codes.html for more information. AUP#CXSNDR ).

It could also be caused by the content of the message. Remove everything from your outbox if there is more than one email in there by moving them to the drafts folder, and resending each, one at a time. when you get the error, you found the offending message. Make sure it's smaller than 17 megs including all attachments, and if it is, and you are still getting the error, it could be treating the message as spam. Try editing the message, removing any hyperlinks in the message and see if that helps