Comcast should offer a way to do that, but they don't.
If it's only happening for one sender, Comcast Security Assurance might be able to help. Give them a call at 1-888-565-4329 (from http://constantguard.comcast.net/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.
Otherwise the only thing you can do is turn off spam filtering completely so that everything they deliver is placed in the Inbox. https://www.xfinity.com/support/articles/spam-filters-and-email-blocking or https://www.xfinity.com/support/articles/spam-filters-and-email-blocking-new-experience shows where to find the setting.