This video explains why the iPhone/iPad do not actually delete from server even when you have them set to do so, until you refresh your device's mail several more times after deleting the message(s). Annoying but true!
http://www.youtube.com/watch?v=SChIy9ycod8
Even with this solution, your mail will not "synch across devices" as such, however when you delete on one it will not later download to the other. If you have already d/l it to the 2nd device it will still be there. But if you d/l on one only (say, your iPhone) and delete, then open mail on an iPad, it will not d/l again if you follow this. It also will no longer be on the Comcast server so if you use Xfinity Connect it will not be there (if not already d/l) and also if you you later d/l on a PC using Outlook or other client, after deleting from the Apple device, it will no longer d/l since no longer on the Comcast server.