mailq.sendmail: 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.sendmail - from package: sendmail

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

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