I empathize 100%. Same communication issues. I resolved it primarily by using the "text agent" on my phone to cox - that is how I got some response - not necessarily that cox resolved the issues I had each time. It was like finding the "secret door"...and the response from widermouth makes no sense to me...if you do not use a ISP email then what? Carrier pigeon?
ISP stands for Internet Service Provider. When we're talking email marketing, ISP refers to the major email providers: AOL, Hotmail, Outlook, Yahoo, Gmail, Comcast, and so on.
Their customers are commonly your email recipients. It's the job of the ISP to protect their customers receiving unwanted or unsolicited emails.