Disappearing recipients when sending to a personal distribution list

A work collegue of mine just explained this strange problem today.

We have an Exchange 2003 server, with Outlook 2003 clients. She creates
on her client a personal distribution list with both internal (we have
a domain) and external addresses; it is fairly small, with 10 to 15
addresses.

When sending an email to this DL, some of the recipients STILL INSIDE
THE DL just disappear from the recipients list of the "To:" field. The
email is regularly sent and saved to the "Sent Items" folder, but the
recipients are missing ALSO in the sent mail (as if the DL somehow
failed to fully expand). We don't have a separate expansion server.

Any ideas?