1) This is from Section 3.3. - Mail Transactions:
If accepted, the SMTP server returns a "250 OK" reply and stores the forward-path. If the recipient is known not to be a deliverable address, the SMTP server returns a 550 reply, typically with a string such as "no such user - " and the mailbox name (other circumstances and reply codes are possible).
2) Regarding RSET specifics - what exactly is the question here?
3) I'm afraid that interpreting the RFCs is beyond the scope of Rebex support. The RFCs themselves are very clear. We would be happy to answer any questions you might have that are related to actual Rebex implementation, but it looks like your support contract has already expired. Please consider renewing it to enjoy continued support.