kmail: 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/bin/kmail - from package: kdepim

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

On Debian possible default full paths are:

/usr/bin/kmail - from package: kmail

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

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