Error when sending to Distribution list

I have a user that created a DL for her department and then sent it to each
user (14 of them). For some reason or another when they select this DL
(which only has 34 contacts in it) they get an error when they click on the
+ to expand it open, which is the following;

Microsoft Office Outlook
Cannot perform the requested operation. The command selected is not valid
for this recipient. Could not complete the operation. One or more
parameter values are not valid.

And if she selects the Send button she gets the following error;

Microsoft Office Outlook
Could not complete the operation. One or more parameter values are not
valid.

Any help is greatly appreciated. We are using OL2003
Penny