O FAQ do sendmail seria um bom começo. (e foi o primeiro hit um pesquisando "sendmail todos os usuários" ...)
There is no magic short-cut for this. But it is not hard to set up: create an aliases entry in
/etc/aliases
alluser: :include:/etc/mail/allusersDon't forget to run
newaliases. Then list your users, one per line, in the file/etc/mail/allusers. You might be able to do that with
awk -F: '$3 > 100 { print $1 }' /etc/passwd > /etc/mail/allusers