checkmail: 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/checkmail - from package: nas

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

On Debian possible default full paths are:

/usr/bin/checkmail - from package: nas-bin

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

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