imapfilter: 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/imapfilter - from package: imapfilter

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

On Debian possible default full paths are:

/usr/bin/imapfilter - from package: imapfilter

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

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