rcpthosts contain the domains that you can send email to regardless of whether your box host that domain or not. locals contain domains that are to be treated locally. This means that on stock qmail, if you have a unix user "david" and your locals contain domain1.com and domain2.com, all emails addressed to
mailto:david@domain1.com and
mailto:david@domain2.com are to be treated as local and sent to the unix user "david"
I personally patch qmail with smtp-auth to take care of relaying. relay-ctrl used to do the job but it allows relays via ip addresses... this is bad news for NAT-based systems as one valid email user inside the network allows all others from that network to use your box as relay.
