mailq: 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 - from package: exim
/usr/bin/mailq - from package: opensmtpd
/usr/bin/mailq - from package: ssmtp

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

On Debian possible default full paths are:

/usr/bin/mailq - from package: courier-mta
/usr/bin/mailq - from package: dma
/usr/bin/mailq - from package: esmtp-run
/usr/bin/mailq - from package: exim4-daemon-heavy
/usr/bin/mailq - from package: exim4-daemon-light
/usr/bin/mailq - from package: masqmail
/usr/bin/mailq - from package: nullmailer
/usr/bin/mailq - from package: postfix
/usr/bin/mailq - from package: sendmail-bin
/usr/sbin/mailq - from package: qmail-run
/usr/sbin/mailq - from package: ssmtp

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

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