Removing old Email addy in Outlook

Hi, I use Outlook Express on my PC's, but have a older friend who uses Microsoft Outlook, and recently she changed her Email addy , and when she sends a Email it is using her old address, and when people try to reply to her they get a error message , that the address is not valid. I have to see about it for her,
I know what to do in Outlook Express but I not familier with Outlook,
also when I send smilies etc on Emails to a Outlook User ( not all), it comes back as a error, would it be their PC set up or maybe their ISP?
Thanks Des.

 

Thanks Steve will check it out when I get down to her place , thanks again Cheers Des.