Numerous people (using Outlook) have reported that they get rejection notices when they Reply to an email that I send from Webmail.
No issues when I originate cox.net emails from Outlook or my Android phone.
Is there a way to prevent this from happening?
I'd suggest going into Settings -> Preferences in webmail and looking through the different options there to see if an alternate reply-to address has been specified and remove it if so. If you do find one and you didn't do it you may want to change your password and secret question/answer options as this activity could be an indicator that your account has been compromised.