in reply to
Email::Send succeeds with "unknown user"
This is the nature of E-mail. Your SMTP server will accept the message for delivery regardless if the recipient exists, because it hasn't even attempted to send it on yet, and this is the only "error" you can retrieve from Email::Send. You'll have to watch for the bounce messages via another method.
Also, if you are using SMTP to the same sendmail ( the one you were forking ) and aren't reusing the connection for multiple messages then you haven't reduce load, you've just changed where you're forking.