When spam filtering is turned on, Comcast does not offer any way for customers to designate "allowed sources". The advice you sometimes see to "put our address in your Contacts to indicate that messages from us are not Spam" doesn't work here. Comcast should provide this, but they don't.
If it's just one or two addresses, their Security Assurance group might be able to fix this for you. Give them a call at 1-888-565-4329 (from https://internetsecurity.xfinity.com/help/report-abuse/). Note that this is NOT one of the general customer service numbers. The first-line reps there are sometimes not very helpful. If so, be persistent, and keep at them until you get a satisfactory answer.
Without their help the only thing you can do to prevent email from going to the spam folder is to turn off spam filtering completely so that everything they deliver is placed in the Inbox.