Update:
I have now provisioned my own email server for flarum notifications. With this configuration, all flarum email addresses are formatted: site-name@mail.linkrobins.com
My software patches these on each website and automates the process. Unfortunately, as is, it overrides custom email addresses (brevo, mailgun, etc).
Taking a break right now. I might get this resolved tomorrow where custom emails are not overrided so the user has options, but right out of the box, emails work.
Fixed it. If a client wants to bring their own email configurations, they'll have to manually replace my credentials and notify me. Afterwards, I can reboot their site with the correct variables to retain their custom email configurations. This is only necessary if you want your email address to include your domain, otherwise, the stock configuration for my hosting will provision your email as mentioned above.
My custom email server brings up 10/10 sending score.