icloud email can only communicate with other icloud accounts (iPhone, iPad and Win 10)

Earlier today, my wife's iCloud email account stopped communicating with any other providers, such as Gmail. Yet it can send and receive from my iCloud email. This is consistent across her iPhone, iPad and Win 10 computer.

This followed a possible malicious attack where she received more than a thousand emails that purported to be a notice of failed mail delivery to a large mailing list that she is on. The thousand emails are deleted but the problems persist. Any ideas?