Email client: usage in e2e tests

[tobiasdiez]

Is your feature request related to a problem? Please describe.
When using the EmailClient to send emails, it's hard to make sure that it works as intended using e2e. A proper e2e test would actually send out an email and check that it is received at a test email. But it may take quite some time until the test email is really delivered, leading to long test times.

Describe the solution you'd like
Provide a way to simulate sending an email, but instead of really sending it return the "would-be-received-email". Alternatively, provide an test email address where one could send emails to, which are delivered immediately and which can be checked easily with an api or similar.

Describe alternatives you've considered
Only manually testing the setup with real email addresses from time to time.