virt_mail-vt: 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/virt_mail-vt - from package: virtuoso-opensource-utils

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

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