mailgraph: 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/sbin/mailgraph - from package: mailgraph

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

On Debian possible default full paths are:

/usr/sbin/mailgraph - from package: mailgraph

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

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