exim4: command not found

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/lib/debug/usr/sbin/exim4 - from package: exim4-daemon-heavy-dbg
/usr/lib/debug/usr/sbin/exim4 - from package: exim4-daemon-light-dbg
/usr/sbin/exim4 - from package: exim4-daemon-heavy
/usr/sbin/exim4 - from package: exim4-daemon-light

You may install the required package with command apt-get install {package}

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