trapmail is a sendmail replacement for unit- and integration-testing that captures incoming mail and stores it on the filesystem. Test cases can inspect the "sent" mails Its commandline aims to mimick the original sendmail arguments, commonly also implemented by other MTAs like Exim or Postfix.
When trapmail receives a message, it stores it along with metadata a JSON file in the directory named in the TRAPMAIL_STORE environment variable, falling back to /tmp if not found. Files are named trapmail_PPID_PID_TIMESTAMP.json, where PPID is the parent process' PID, PID trapmails PID at the time of the call and TIMESTAMP a microsecond accurate timestamp.
While trapmail avoids collisions between stored messages, it cannot guarantee that other test processes/threads that are running simultaneously do not add to the store as well. Care must be taken to avoid race conditions, especially when clearing the mail store.
Providing different TRAPMAIL_STORE targets allows for namespacing the data. Otherwise, depending on the usecase, PID and PPID may aid in filtering.
The trapmail crate comes with a command-line application as well as a library. The library can be used in tests and applications to access all data that trapmail writes.