mailexec: 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 Debian possible default full paths are:

/usr/bin/mailexec - from package: mailavenger

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