exim: 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 CentOS possible default full paths are:

/usr/sbin/exim - from package: exim

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/sbin/exim - from package: exim4-daemon-heavy
/usr/sbin/exim - from package: exim4-daemon-light

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

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