mailq.ssmtp: command not found

Have You tried accessing the binary with the full path?

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.ssmtp - from package: ssmtp

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