formail: 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/formail - from package: procmail

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

On Debian possible default full paths are:

/usr/bin/formail - from package: procmail

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

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