It is reported fairly often. Comcast should provide a way users can whitelist a sender so their mail would not be considered spam, but they don't. And unfortunately, the "Not Spam" function takes a lot of clicking on a lot of messages to perhaps stop future messages from being filtered.
If @ComcastAntiSpam or another employee doesn't respond here in the Forums, Comcast Security Assurance might be able to help if it's just one or two addresses. You can reach them 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, so you may need to be persistent and keep at them until you get a satisfactory answer.
Without Comcast's help or if this is happening with a large number of senders and "Not Spam" doesn't work, the only thing you can do to prevent messages from going to the spam folder is to turn spam filtering off completely so that everything they deliver is placed in the Inbox.
