sb_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 Debian possible default full paths are:

/usr/bin/sb_imapfilter - from package: spambayes

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

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