Nigel,
Could you clarify a few more details like:
What kind of emails SPAM, NDR's etc / Are you hosting an exchange server if not what provider are you using for emaili.e. yahoo.com / msn.com etc
There was a virus which is known to do this, I have known several people who have had this issue with hotmail, If this is the case I would begin by doing a virus clean up on your pc to make sure your not infected and change all your login security settings. The issue more then likely is your friends accounts have been compromised what is the nature of these emails??
