checkgmail: 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/bin/checkgmail - from package: checkgmail

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

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