re-directing root's mail to my external mail id

[David Jansen]

Others already pointed out that the aliases file will probably work well
for what you want.
Just an additional note: sendmail is somewhat paranoid about .forward
permissions (and rightly so): if a .forward is world writable or the home
directory is, it will refuse to use it, and some message about this will
end up in your system logs. Maybe that's where the difference between
your systems was.

David Jansen