Geez, receiving a couple marketing emails would exceed 250kB. Maybe if you were using a text only email client and doing nothing else, and you only get emails from close friends and family, that could be sufficient...
I guess you'd also have to be using a non-smart phone, or you'd have to go in and disable all telemetrics and similar features of every app you used...
All I could find on a quick, cursory search was this, from their ToS:
I don't have Comcast, so I can't get any further details.