This file contains all of the domains your server will accept mail for. Using the VqAdmin interface or the command line tools will automatically add the domains you create to this file.
This is where you want to put domain names that you want to be a secondary mail server for. Do not add the domains or users if you want to be a caching email server. If you add a domain here that does not reside on your server, the machine will accept the emails for that domain and attempt to deliver them to the main mail server (main MX record) every 5 minutes until the value in queuelifetime is met or they are successfully delivered. Once the queuelifetime has been exceeded, the messages will be bounced to the originating sender.
man qmail-smtpd excerpt:
Allowed RCPT domains. If rcpthosts is supplied, qmail-smtpd will reject any envelope recipient address with a domain not listed in rcpthosts.
Exception: If the environment variable RELAYCLIENT is set, qmail-smtpd will ignore rcpthosts, and will append the value of RELAYCLIENT to each incoming recipient address.
rcpthosts may include wildcards:
Envelope recipient addresses without @ signs are always allowed through.