mailq.exim: 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/bin/mailq.exim - from package: exim

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

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