Even though the contact is in the Address Book, typing the contact name into the To box does not resolve to the e-mail address, causing the send to fail.
This behavior will occur if you are using Windows XP SP2 and the Locked Down Value Name is set to disable Outlook Express identity switching.
To resolve this behavior, enable identity switching:
1. Open a CMD.EXE window.
2. Type the following command and press Enter:
reg add HKCU\Software\Policies\Microsoft\Windows\CurrentVersion\Identities /V "Locked Down" /T REG_DWORD /F /D 0