I am not receiving all of my emails

I have had several people tell me that they emailed me but I have not received the emails. My spam and junk folders are empty. In at least one instance, as of 6 weeks ago, I had been getting several emails a week. They tried to send me a test message today and it did not come through and did not bounce back to them. that email address is artguide@artguidenow.com

I don't know how many other emails are not coming through which concerns me. What else can I check or is it possible they are stuck on a server? Also, is there a place I can whitelist domains?