mailmail: 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/mailmail - from package: python-twisted-mail

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

On Debian possible default full paths are:

/usr/bin/mailmail - from package: python-twisted-mail

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

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