update-exim4.conf: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/sbin/update-exim4.conf - from package: exim4-config

You may install the required package with command apt-get install {package}
Error in other languages:
update-exim4.conf: bevel nie gevind nie
update-exim4.conf: comandă negăsită
update-exim4.conf: comando não encontrado
update-exim4.conf: commande introuvable
update-exim4.conf: command not found
update-exim4.conf: käsku ei ole
update-exim4.conf: không tìm thấy lệnh
update-exim4.conf: komanda nerasta
update-exim4.conf: Kommando nicht gefunden.
update-exim4.conf: kommandot finns inte
update-exim4.conf: komut yok
update-exim4.conf: nie znaleziono polecenia
update-exim4.conf: níor aimsíodh an t-ordú
update-exim4.conf: no se encontró la orden
update-exim4.conf: no s'ha trobat l'ordre
update-exim4.conf: opdracht niet gevonden
update-exim4.conf: parancs nem található
update-exim4.conf: perintah tidak ditemukan
update-exim4.conf: príkaz nenájdený
update-exim4.conf: příkaz nenalezen
update-exim4.conf: команда не найдена
update-exim4.conf: командата не е открита
update-exim4.conf: 命令找不到
update-exim4.conf: コマンドが見つかりません