I followed the docs at https://laravel.com/docs/5.8/mail
and set up Mailgun working perfectly through the sandbox, sending emails as developed.
Moved to production, added my domain to mailgun. Set up the MX records, etc. Under the DNS section at Mailgun, all the settings have green checkmarks.
The mailgun domain is set to my domain, not the sandbox, and the secret is set to the correct key.
I sent an email programatically. It went through, but was sent through the sandbox, not the production domain.
I don't understand how this is possible.