This was an oversight so far that became apparent after our
recent fix for email addreses with query params
(e.g. `test@example.com?subject=test`).
The parsing of email addresses has improved and so we detect
more mail addresses, but we didn't check if they belonged
to an example domain, causing false-positive checks.
Unfortunately it's not possible to automatically enable features
for `cargo test`. See https://github.com/rust-lang/cargo/issues/2911.
As a workaround to allow for using example domains for unit- and integration
tests, we introduce a new feature, `check_example_domains`, which is
disabled by default for normal users. The feature gets activated for the
integration test which checks that the example domain exclusion works as
expected.
