OK,
I have found a way around this problem (at least this works for me):
# Only set organisation.isp.com as the main domain, no virtual domains.
# Create a user called '' (or whatever your main account is called)
# Create a user called 'adminmail'
# Redirect all administrative mail to adminmail@organisation.isp.com
# Install user-manager from myezserver.com
# log onto user-manager as adminmail
# Go into process mail
# Create a rule that says forward all mail from root@organisation.isp.com to the user
# Change to mode to 'geek' (this didn't work for me, I had to make a copy of the .procmailrc file, disable procmail, enable it again in procmail and restore the copy to .procmailrc).
# Edit the .procmailrc file and change the line that says:
* ^From.*root@organisation.isp.com
to
* !^From.*root@organisation.isp.com
(i.e. insert the '!')
Now all mail from the admin account will go to the account, execpt stuff send from the root account.