messages from unknown sender

Hi,
We’re using Exchange 5.5, sp4 and Trend Micro virus scanning which shows
no viruses in our system. One of our users received a message from
someone they don’t know and they said she sent them a message with a
suspicious attachment. She doesn’t know this person or their email
address yet they forwarded a copy of the message she allegedly sent to
them. It definitely had her email address but she didn’t send it. It
looked like a Klez variant (the subject was “How are you”). Any ideas on
how this happened?

Thanks.