mtest: 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/mtest - from package: uw-imap-utils

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

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