mimepostcard: 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/mimepostcard - from package: perl-MIME-tools

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

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