smtp recipient verify. This qmail-smtpd feature will look up every RCPT TO address in ldap with qmail-verify to check the recipients existance. If it does not exist, qmail-smtpd will answer directly with a 550 reply instead of accepting the email and bouncing it later. Only addresses whose domain part is listed in ~control/locals are checked (because only there we know the definite answer). Relaying and rcpthosts works as before, recipient is not checked for those.
Some thoughts regarding this:
I can see quite a few situations where you would want to be able to check recepients when relaying/routing as well.
One common example would be a secondary MX, that receives mail for a domain if the primary MX is down.
The domain would not be in locals on the secondary, but in rcpthosts.
You would still want to be able to bounce non-existent recipients immediately on the secondary, rather than queue all mail for a domain until the primary is up and gives the secondary a 550.
As a lot of spamproxies and viruses go for secondary MXs directly, even if the primary is up, this is a regular issue, not just something that happens when there is a problem with the primary.
Also, I don't think that it's uncommon to have the servers handling incoming email separate from the final mail store servers, with smtproutes for domains in rcpthosts specifying where the mails should finaly be delivered.
You might not want to verify recepients for all domains in rcpthosts though, so checking rcpthosts rather than/as well as locals would not be a solution.
Something like the checkdomains control file that was used in Ricardo Cerqueira's rcptcheck.patch would be a possibility though.
Patrik
