Hehehe, yeah.
Seriously though, do you have an smtp server on that box?
I'm not seeing one in the Dockerfile or any of the docs, so you'll probably need to use external smtp, it's in the osTicket admin settings for your primary email account.
Odd that the test worked but other email doesn't.
If nothing in the admin logs, then you'll need to check the webserver logs, but they're generally off by default (change in main.inc.php and php.ini).
You might also need to enable header spoofing.. not sure, always had postfix myself.