Postfix, virtual file

Похоже речь о postfix еще 1-ой ветки.

man 5 virual
...
POSTFIX-STYLE VIRTUAL DOMAINS
With a Postfix-style virtual domain, the virtual domain
has its own user name space. Local (i.e. non-virtual)
usernames are not visible in a Postfix-style virtual
domain. In particular, local aliases(5) and mailing lists
are not visible as localname@virtual.domain.

Use a Sendmail-style virtual domain (see below) if local
usernames, aliases(5) or mailing lists should be visible
as localname@virtual.domain.

Support for a Postfix-style virtual domain looks like:

/etc/postfix/virtual:
virtual.domain anything (right-hand content does not matter)
postmaster@virtual.domain postmaster
user1@virtual.domain address1
user2@virtual.domain address2, address3

The virtual.domain anything entry is required for a Post-
fix-style virtual domain.

Do not list a Postfix-style virtual domain in the main.cf
mydestination configuration parameter. Such an entry is
required only for a Sendmail-style virtual domain.

With a Postfix-style virtual domain, the Postfix SMTP
server accepts mail for known-user@virtual.domain and
rejects mail for unknown-user@virtual.domain as undeliver-
able.
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В твоем случае "включается" postfix-стиль виртуальных доменов и вся почта для домена domain.ru направляется локальному пользователю admin.