postmark: 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/postmark - from package: postmark

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

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