three changes required -

• configure sendmail to use a remote host for all mail in /etc/mail/submit.cf

    # changes to fwd mail directly to smart host
    #D{MTAHost}[127.0.0.1]
    D{MTAHost}[smtp.muse.net.nz]

• configure local aliases mapping to remap users to a destination address in /etc/mail/aliases

    # Well-known aliases -- these should be filled in!
    # root:
    root:           root@muse.net.nz

• permit relaying on smart host (postfix in my case) in /etc/postfix/main.cf

    mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain, akai.$mydomain