imp-mailbox-decode: 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/imp-mailbox-decode - from package: php-horde-imp

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

On Debian possible default full paths are:

/usr/bin/imp-mailbox-decode - from package: php-horde-imp

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

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