Hello, @lixqpxil

I can’t give you specific instructions because I’m not familiar with the server configuration and etc. However I can give you some basic instructions in order to troubleshoot this.

Have you checked the PHP error logs for your sites to see if there is an error generated when you execute the send email script?

Did you made any changes to the postifx configuration after you’ve installed it? If you’re not sure that postfix was installed successfully, you can follow this tutorial:

https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-postfix-on-ubuntu-18-04

Hope this helps!

Regards,
Alex