Syncthing is one of the best tools for keeping folders synchronized across the internet. However, for one or the other reason, it occasionally stops to work on my Ubuntu 16.04 and because I use it in a "set it and forget it" fashion, I also forget how to check that it is operational.
Make it start automatically at boot time (as a system service):
systemctl enable email@example.com
Start the service:
systemctl start firstname.lastname@example.org