Can’t Access E-Mail using Outlook or MacMail (Error Code 3170)
I finally found the solution. It wasn't a Cox problem, nor was it an e-mail client problem. Turned out to be a 3rd party software issue.
On the Apple discussions forum, it was suggested to download the EtreCheck diagnostic app for Mac; it paid off. The problem turned out to be the following “Unsigned” file: com.DataFormatSearchP.plist. I have no idea what an “Unsigned” file is, or what this one did, but deleting it fixed my problem. Mac Mail is finally working!
Thanks for everyone's replies.