Q: I can receive most of my email in my @icloud.com account but not from my wife or certain other people. My wife also uses @icloud.com account.  What is happening?

I receive some but not all email sent to my @icloud.com account. For example, I receive nothing from my wife, who also uses an @icloud.com account. This has happened many times in the past. How can I resolve it? I have followed all of the suggestions by Apple.