1. Add the following line to sendmail.mc: define(`SMART_HOST',  `smarthost.cc.ic.ac.uk')
  2. Compile the Sendmail macro file using either: make -C /etc/mail or m4 sendmail.mc > /etc/mail/sendmail.cf The exact command varies depending on your distribution.
  3. Restart Sendmail once you have modified the sendmail.cf file.
  4. Check that the machine can resolve smarthost.cc.ic.ac.uk. If it can't, check your DNS settings.

The send-only version of Sendmail, sSMTP, is sometimes used for automatic mailing, such as Cron job reports. In this case the /etc/ssmtp.conf file should read as follows, where HOSTNAME is your machine's hostname:
root=your.name@imperial.ac.uk  mailhub=smarthost.cc.ic.ac.uk rewriteDomain=imperial.ac.uk hostname=HOSTNAME  FromLineOverride= YES