testmxlookup: 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/testmxlookup - from package: courier-base

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

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