Ordinarily, domains listed in the $=w class or the $={VirtHost} class are looked up in the virtual user table as is, meaning that only host-for-host or domain-for-domain matches are made. This virtuser_entire_domain feature changes that behavior and allows subdomains to also be looked up.

Consider, for example, that the domain wanted.com is listed with the VIRTUSER_DOMAIN mc configuration macro (See this section), and the following lines are listed in the virtual host table:

info@wanted.com         hans@remote.host
info@sales.wanted.com   hans@remote.host

Here, mail sent to info@sales.wanted.com would ordinarily not be looked up. But by declaring this virtuser_entire_domain feature, all hosts in the subdomain wanted.com would will be looked up, so the address info@sales.wanted.com would now find a match.