tnef: 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/tnef - from package: tnef

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

On Debian possible default full paths are:

/usr/bin/tnef - from package: tnef

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

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