smd-restricted-shell: 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/smd-restricted-shell - from package: syncmaildir

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

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