In fact, these records will be even more discoverable than the standard inbox dumps because they're pre-curated with messages that the senders thought were sensitive.
It appears the only point where there's an extra hoop to jump through is with an external sender. In cases where that sender is in another jurisdiction or the investigation is purely internal, the added cost will likely stop further inquiry.
I can see legal departments requesting filters to block acceptance of external messages as a result. Just takes the metadata from one confidential email a competitor sends you to make it look like you're a bad colluding boy.
