Dropping the connection with mere TCP RST (or whatever it is) is fine by me. Accepting the message, saying 200 and then not delivering it (not even to the "spam" folder) — that's what I feared.
We used to have issues sending password reset emails to users with Outlook hosted email - Outlook would return a 250 status code, then silently discard the email. It never turned up, not even in the spam filter.