Email not receiving if multiple addresses are used by sender

I have had an Apple email account for over a decade. I have used it a lot. Especially for family events. I sent out a group email to family members trying to organize an event. They have various emails, some yahoo, some gmail, some att. For whatever reason, they cannot reply to my email. They get a "failure to send" message. This is a huge problem for me as I imagine it would be for many others. If I cannot use this group email and receive replies the apple account becomes useless to me. Why all of a sudden after all these years is there a problem? I have sent myself singular emails from different accounts and it works. So, what is the problem?