I cannot receive a friend's email

My friend and I have been emailing daily for years now. Suddenly, I cannot receive her emails anymore. She can receive mine though. Both our emails are working in that we are sending and receiving from others. She has troubleshooted her end with her ISP and I being that I have a .mac email account, I have used the forums and internet to troubleshoot my end. Nothing is working. I have tried a thousand different things. She is not blocked. I have no rules applied that would filter her. She is not going to my junk mailbox. I even checked my email out on the server to see if her email was making it that far and then just not coming into my inbox. Her email is not reaching the server. Her ISP says she is not blacklisted. So...what then? She was able to email me from her husband's email address yesterday and occasionally, I will get a random (days old) email from her (from her email address). We cannot figure out what is going on. Can someone help me? Thank you.