A friends old email address keeps popping up

I receive group emails and one person changed her email address. I have changed it in my contacts and have removed the old address. I think because others in the group use the old address when including me in a group email. Then my email program starts using the incorrect address again when I initiate emails to her, even though it's not in my contacts. Is there a way I can block that email address even though other people send me group emails using the wrong address for this person? This is happening on iMac, but probably phone and iPad also. Thanks