Safe domain keeps going to spam

There's a certain domain that previously would come thru to our inbox, at some point something changed and now any emails from that domain are sent to spam. I've tried to create a filter rule, but that doesn't work either. The emails are important and time sensitive and keep getting lost in the 100s of spam emails we receive. How can we get them to deliver back to the inbox? Is there something on their end they can do or can comcast do something?