Can't Send Email from Shared Contacts Folder

Outlook 2007 & Exchange 2003
======================
Hello,

I created a sub-contacts folder in my folder list and shared it with another
Outlook 2007 user in our group by right-clicking on the sub-folder and
selecting Share... and checking the option for them to add, edit, etc.

They received the email and clicked on open and they were able to see the
contents of my sub-contacts folder.

However, when they right click on a contact in the shared folder and select
Create...new message to contact, fill out the email and then click on Send
they receive a "Cannot Send Item" message and are unable to send the email.

Any thoughts on why they are receiving this message?

Thanks for any insight you can pass along.