mailq.opensmtpd: 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.opensmtpd - from package: opensmtpd

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

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