dbmail-pop3d: 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-pop3d - from package: dbmail

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

Error in other languages:
dbmail-pop3d: bevel nie gevind nie
dbmail-pop3d: comandă negăsită
dbmail-pop3d: comando não encontrado
dbmail-pop3d: commande introuvable
dbmail-pop3d: command not found
dbmail-pop3d: käsku ei ole
dbmail-pop3d: không tìm thấy lệnh
dbmail-pop3d: komanda nerasta
dbmail-pop3d: Kommando nicht gefunden.
dbmail-pop3d: kommandot finns inte
dbmail-pop3d: komut yok
dbmail-pop3d: nie znaleziono polecenia
dbmail-pop3d: níor aimsíodh an t-ordú
dbmail-pop3d: no se encontró la orden
dbmail-pop3d: no s'ha trobat l'ordre
dbmail-pop3d: opdracht niet gevonden
dbmail-pop3d: parancs nem található
dbmail-pop3d: perintah tidak ditemukan
dbmail-pop3d: príkaz nenájdený
dbmail-pop3d: příkaz nenalezen
dbmail-pop3d: команда не найдена
dbmail-pop3d: командата не е открита
dbmail-pop3d: 命令找不到
dbmail-pop3d: コマンドが見つかりません