The first two lines say that mail arriving at your site, regardless of its origin, will be
allowed to be relayed to bob at the site friend.domain (the first line) or to any user at
the site friend.domain (the second line). This provides a way to allow relaying on a
host-by-host basis, even if you have all relaying turned off with the various antirelay
features (?§7.4 on page 267). This is useful if you are a secondary MX site for
friend.domain.
The second line says that we will also relay mail from the site friend.domain. A line
such as this requires that you have declared the FEATURE(relay_mail_from) (?§7.4.8 on
page 274) witha literal domain second argument. Here, if the envelope sender is any
user at friend.domain, the envelope recipient can be a local or remote address.
The third line says that we will specifically accept connections from the host
friend.domain. We do this because that site might be rejected if it is listed on some
DNSBL site. An OK via a Connect prefix overrides any rejection based on DNSBL
lists.
The fourth line rejects connections from the host bad.domain no matter what. This is
one way to reject connections on a site-by-site basis, if, for example, you want to block
messages from a site that is pushy but not eligible for listing with a DNSBL server.
Even if you lack an immediate use for these prefixes you should consider using them,
just to experience their power.
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