Julie, I ran into the problem you are having replying to messages
before. It isn't a new problem. I found this reference to it in
"INSIDE OUTLOOK EXPRESS"
http://www.insideoe.com/problems/error.htm#line3 Copy follows:

Malformed message ID: Line 3 References too long
This error message occurs when you reply to a deep thread in newsgroups.
Line 3 is the References header. It contains the message ID of every
message in the thread. When this line becomes too long, Outlook Express
adds a carriage return instead of removing some of the IDs in the
middle. This makes the last message ID invalid and so your reply is
blocked from being sent.
To work around this bug, save your reply to your desktop by clicking
Save as on the File menu. Open the saved *.eml file in Notepad and edit
line 3, References, by removing some of the message IDs in the middle.
Be sure to leave the first and last IDs so that your reply will thread
properly once it is posted. Saved your edited file, then double-click it
to open and click Send.
An alternate method is to save, not your reply, but the message to which
you want to reply. Edit the saved message to remove some of the message
IDs, save the changes, then open the edited file and click Reply.