If wordpress mail function doesn’t work, you might need to install sendmail package into server.
sudo apt-get install sendmail
Thanks to:
https://stackoverflow.com/questions/8803994/php-mail-not-working-for-some-reason
https://askubuntu.com/questions/937666/ubuntu-16-04-command-line-sendmail-installation-hanged
https://wordpress.stackexchange.com/questions/95016/wordpress-refuses-to-send-mail-your-host-may-have-disabled-the-mail-funct