I've been thinking about finding the cycles to whip out a utility that would take a local_part on the command line; and return success or failure to indicate whether it is deliverable. It would work by starting an LMTP session and issuing a RCPT; then RSET and QUIT after getting the response to the RCPT. Having to spawn a task to do that is a bit ugly; but I don't think Exim has a way to do it internally. (Or at least not directly. I might be able to do something with the built-in perl lib...)
Follup: I mistyped, that should have been "verify = recipient/callout" in your ACL, otherwise Exim will only verify that you have a router that will accept the address, not that that it's actually deliverable.
