Mail isn't being sent

I have been unsuccessful in the email test. I do not get an error, and there isn’t anything in the log files.

I also tried adding an author, and sending the invite, also no email received.

Ghost 3.2.0
mail is setup as direct (from the default install)


Installed on a Digital Ocean droplet (I followed the instructions on ghost.org for installing on ubuntu 18.04 LTS)
There are no errors reported in the console.