incoming email from a family member at metrocast.net is blocked before reaching my comcast.net mailbox.

Email from a family member @metrocast.net is blocked before reaching my comcast.net mailbox. (It does not arrive in my inbox or junk folders.) The situation occurs periodically - every month or so - and persists for days before eventually returning to normal. If, during the periods when things don't work properly, the sender sends a message to both my comcast.net and gmail.com addresses, it arrives @gmail.com but not @comcast.net. I suspect the email is getting caught in a spam filter somewhere. How do I fix this?