email errors in outlook

I had a Hotmail email account, but then I read that Microsoft was going to stop Hotmail and just go with
outlook. So I reg. for a outlook account and then made outlook my main email account. As I getting all of my emails together, I had a problem with one email address. When I input the persons name and email
address into my contact list and click save, outlook will save it, but then if I try to forward a email to
that person and I click on his contact info, outlook will put the persons email address in the email box, but when I click send it goes out, but then comes back a few min. later as not delivered. Now here is the funny thing in my contact list if I click on any other persons email address when it comes up to view it, it is in black writing, except for that one person there email address comes up in red writing. The only
other thing I find is that I am using outlook instead of outlook express, one is the free version and the other one is paid.