One of the SMTP tests was doing an external DNS lookup for a
non-existing host, which is reasonably harmless but makes the test less
hermetic.
This patch changes the non-existing host for an invalid address, which
has the same effect but avoids the network lookup.
This is a hack; ideally we would be able to override the resolver, but
Go does not implement that yet, and changing the code is not worth the
additional complexity.