Problem with Distribution List

A client has set up distributions lists and when they type in the name it shows in the to: box and when you check names it appears as normal. They are attaching a small pdf file and when they send the message it doesn't allow for it giving an error of the parameters being wrong.
However when they select the distribution list from the contact list instead of typing it in and add it to the to: field and add the attachment it goes out fine.
They want the short version of typing the list name in instead of having to select it.
They are using a laptop with XP Pro and Office XP w/business contact manager.