550 User Unknown

[Herbie]

Am I right in think qmail-smtpd will never give this error? 

As I see it qmail-smtpd will only check the domain of the message to see
if it is valid, ie it will accept any valid username for localhost whether
the user actually exists on the system or not. 

Is that part handled by qmail-send?

Herbie (not a C guru but interested)