Can I upload legacy email data?
Yes using the historical data upload app, you can ingest legacy email data in PST, EML, MBOX files and from IMAP accounts hosted on mail servers.
Does retention policy of a Vaultastic plan apply to legacy data?
If you upload legacy data to a retention based plan (Continuity, Tracer or Durability), then the retention will be applied on the legacy data uploaded to these accounts.
However, when legacy data is uploaded to a storage based plan like Hold, no retention is applied.