exiwhat: 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/exiwhat - from package: exim

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

On Debian possible default full paths are:

/usr/sbin/exiwhat - from package: exim4-base

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

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