Comcast doesn't offer a way to whitelist an address. They should, but they don't. Strangely, they have for many years offered a way to say "I ONLY want to accept emails from addresses on this Safe List, and reject all others". I imagine that facility is useful to almost no one, whereas a whitelist would be useful to many.
The only way I know of to "whitelist" an address without Comcast help is to
Recently announced "whitelist" function: https://forums.xfinity.com/t5/Email-Web-Browsing/Address-Book-Exemption/m-p/3368877#M220644.
You can whitelist EVERYBODY by turning off spam filtering completely, so that everything is delivered to the Inbox.
If it's just one sender, Comcast Security Assurance might be able to help. 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.
I imagine that facility is useful to almost no one, whereas a whitelist would be useful to many.
Not to go off topic, while a whitelist/blacklist function would be nice, the Safe List does have a use or two. Since Comcast email accounts are predicated on the "family/household" model, it would be useful for parents wanting to give a minor child a restricted email account to use only with family and schoolmates, etc. I can also see where it could be used for limited use projects from work or school where only a set list of people are involved. Its an alternative to paid subscription email services that cater to parents of young children.