expn: 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/expn - from package: sendmail-base

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

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