Mail not including all emails in Contacts group

Each attempt to Send Email from a specific group always omitted the same member of the group. The omitted member had a yahoo account and selecting the individual outside the Contacts always was successful.

In examining each member of the group in the Contacts Group list, the email of the member being omitted had the email address dimmed. There was a small icon to the right (I don't recall what it was) and clicking it asked to add to the contact. Answering Yes resulted in the email address no longer dim and the next attempt to email from that group in Contacts included all members of the Group.

I searched quite often and stumbled across this by accident. I hope it is of some value to others experiencing this behavior.