man qmail-send excerpt:
List of virtual users or domains, one per line. A virtual user has the form user@domain:prepend, without any extra spaces. When qmail-send sees the recipient address user@domain, it converts it to prepend-user@domain and treats it as local.
A virtual domain has the form domain:prepend. It applies to any recipient address at domain. For example, if
is in virtualdomains, and a message arrives for email@example.com, qmail-send will rewrite the recipient address as firstname.lastname@example.org and deliver the message locally.
virtualdomains may contain wildcards:
.fax:uucp-fax :alias-catchall .nowhere.mil:joe-foo-host
virtualdomains may also contain exceptions: an empty prepend means that domain is not a virtual domain.
qmail-send handles virtualdomains after locals: if a domain is listed in locals, virtualdomains does not apply.