Not all group members receive email

I have a client who is piloting Domino and Notes. He has created a group on his local machine which contains 4 names. The names are all contacts which show up in his personal address book and each of the names has an internet address. None of the names have Lotus Notes addresses.

When he addresses an email to this group and sends the note he sees the message at the bottom of the screen which indicates that the mail was submitted for 4 people/groups. However, only three of the people actually receive the note. The first name in the list does not receive the note. Upon further investigation the Domino Log in the Mail routing events shows that the note was delivered to the last three entries in the group but not the first, even though the client has indicated that the note was sent to 4 people. Also, the note is not sitting in mail.box awaiting delivery.

Has anyone seen something similar?

Thanks for any and all assistance.