Yahoo plus looks to be an email service. We all know that because of spammers, sending multiple copies out is now being locked down to outright denied. But reading your post I can't tell what email software was in use.
Try to tell more.
I just purchased a Sony Vaio All-In-One 24 inch Desktop and upgraded the Vista with Windows 7 Ultimate 64bit. I cannot forward in any category multiple e-mail addressees from my contact lists. I am using Yahoo Plus. When I hold down the ctrl key I can check about 6 contacts and then the system deletes all of the checked contacts and accepts only one contact. Yahoo will not respond and I don't know what to do. Please Help