calibre-smtp: 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/bin/calibre-smtp - from package: calibre

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

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