dbmail-users: 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/dbmail-users - from package: dbmail

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

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