[andix]

Easy REST API access can be a quite useful feature too.

For automated integration testing it's a must. The test can verify in the end if the expected emails were sent out.

I think Mailpit can even be set up as a real SMTP server, handling a (sub) domain. Either as a MX or just via forwarding rules. Sometimes it can be useful to periodically run integration tests on a production system. So your tests could create accounts based on your test domain ([e-mail]), which is deliverable from every email server, and the tests can verify the delivery. An automated script can then periodically delete the *@testsystem.company.tld accounts.