Your requirement that ...there is no need to use any 'mailbox' formats... is, AFAIK, of no consequence since the single file storage format is a function of sendmail - mail readers know that the e-mails in the mailbox file are stored as sequential ASCII text blocks - at least on *NIX anyway.

I don't know and care less about the format used by Lookout (errrm, Outlook)/Exchange - but I dare bet it's in some weird proprietary binary format...