Paul wrote:

The Read receipts don't exist (at least visibly to me) in the outbox, so I
don't think this would work.

It's guaranteed to work and you won't see read receipts because their hidden
items. They're still in the Outbox, though, and that's why creating a new
PST (with a brand new, empty Outbox) and making it your delivery location is
guaranteed to work.
--
Brian Tillman