The processing what Mireka does on a mail is mostly determined by the filter chain. The filter chain consists of filters. For example AcceptGlobalPostmaster is a simple filter, which can be used to make sure that an email sent to the special postmaster recipient is accepted without further checks.
A single filter is called in each step of the mail transaction. For example it is called when the client sends the reverse path (sender) of the mail. Later it is called again when the client sends the address of a recipient. Each time, the filter does some processing and then optionally calls the next filter in the chain. It can also do some processing after the filters following it completed. The result of the processing can be, for example, the rejection of a recipient. In another case, the processing can be the relaying of the message to the backend server.
For the list of available filters, see the All Known Implementing Classes section of FilterType.
The filter chain is configured in mx.js and
submission/submission.js.