There is no setting for you to change on the Comcast side (that I know of). You have to go to the setting on your phone. Look for a term "sync." By default "sync" is automatically on. You would have to change it to "off."
I am going to make a suggestion to you, though---I and many other Comcast Customers have had an on-going, lengthy problem of NOT getting any new email, whatsoever, unless we turn our phones completely OFF; then having to turn our phones back ON. If we did not go through that--we would never, ever get any more e-mail. There are a LOT of problems and bugs with the Comcast E-mail. Server outages, Sign-in server outages, and on and on. You are automatically getting new e-mails--I suggest that you keep it that way, for now.----Just so that you AVOID problems.