I am facing the same issue. Still; I encountered the same problem a year ago but wasn’t able to trace this back to the email settings. ([solved] Restoring config.json fails)
If I set SendEmailNotifications=true the server fails to start. (It hangs after pinging the db.) If I set SendEmailNotifications=false the server starts just fine. This is very frustrating. My mattermost.conf is created by gitlab.
My config.json validates with all linters that have been used above, so please don’t answer with another “your json is invalid”. This is clearly a bug in Mattermost.