Most likely your mail is sent by the server your Flarum is installed on. To be honest, Flarum sends a request to deliver mail to a recipient, but does not handle that request itself. There is some intermediary (mailgun, local server etc) that has this responsibility. In your situation your local server is processing the request and most likely is not configured to send mail on your behalf (SPF records - best thing ever) or is blacklisted by another domain on the server (now or in the past).
Related to SPF records, gmail and most other mail providers are marking mail as spam when the originating server is not included in the SPF record of your domain.
