Handle `Von: ... im Auftrag von ...` correctly

Currently, when the "From" line in the forwarded email is in the following format, the sender's email is resolved incorrectly as the full string after `Von:`:

```
Von: John <john@example.com> im Auftrag von Jane <jane@example.com>
```

Not sure exactly how, but it would be great if we could try and extract the correct value here.