rt-email-digest: 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/rt-email-digest - from package: rt3

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

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