sb_unheader: 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_unheader - from package: spambayes

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

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