I don't know if Zeit Now (while really cool!) would work for NodeBB... NodeBB is a web application, and does lots of stuff in the background that relies on a server running. To have a service that spins up whenever a request comes in would be much too slow, if only because that first spin up would take many seconds!
@PitaJ and who else may encounter this issue, here is how I solved it.
The root problem was the image format used for "Site Logo" in general/settings.
I've uploaded a png file and the information was not saved in the database, althoughthe fields remained filled across the sessions.
Once I've changed the site logo with a jpg file, the emails were properly formatted too.