exim_lock: 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/exim_lock - from package: exim

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

On Debian possible default full paths are:

/usr/sbin/exim_lock - from package: exim4-base
/usr/lib/debug/usr/sbin/exim_lock - from package: exim4-dbg

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

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