Currently, the Outlook extractor fetches the "Accepted By Mail Server" date from POI, and then saves this into Metadata.EDIT_TIME and Metadata.LAST_SAVED, neither of which look right, and neither of which are date properties.
The rfc822 parser uses Metadata.CREATION_DATE, which is a Date property. The mbox parser uses Metadata.DATE, another (but different) Date property
All three should probably use the same. I'd suggest that for now, they all output the same value to both CREATION_DATE and DATE