In the LuckyBackup GUI, click on Profile > Email. Change the command in the first dialog box to what you want it to be. I don't know of a sendemail command. I have LuckyBackup installed, and there's no script for it, either. The closest thing I can find on my system is /usr/share/vim/ftplugin/gitsendemail.vim, which is a Vim plugin, not related to LuckyBackup.
Note, I've never tried the email function in LuckyBackup.