postdrop: 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/postdrop - from package: postfix

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

On Debian possible default full paths are:

/usr/sbin/postdrop - from package: postfix

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

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