"Cannot forward" doesn't give us much to go on. What happens when you try? What kind to forwarding are you trying to do? What hardware, operating sytem, and program or app are you using? If using Comcast's webmail, is it the older "Zimbra" ("web.mail.comcast.net/zimbra/..."), or the newer Open-Xchange "Appsuite" ("connect.xfinity.com/appsuite/..."), and which browser?. Any error messages?
The more details you can provide, the better, but be aware that the forums are public and it isn't safe to post personal information here. It's also against forum rules -- see http://forums.xfinity.com/t5/Forum-Community/Forums-Policy-and-Guidelines/m-p/2618379#M1.