Comcast doesn't seem to let me forward "bulk" mail that I receive. I had "all mail" forwarded, yet Comcast categorizes some of the mail I receive as bulk mail and won't forward it! Not convenient!
They only forward mail they deliver to the Inbox. If you want to also forward messages they consider to be spam, you'll have to turn off spam filtering. If you have the new Open-Xchange "Appsuite" webmail ("connect.xfinity.com/appsuite/..."), you can do that by clearing the checkbox next to
(gear icon) / Settings / Mail / Advanced Settings / Spam Filtering / Enable spam filtering