sendemail: 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 Debian possible default full paths are:

/usr/lib/prey/platform/linux/bin/sendEmail - from package: prey
/usr/bin/sendemail - from package: sendemail

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

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