checksendmail: 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/sbin/checksendmail - from package: sendmail-base

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

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