We now have enough to understand the lefthand side. This rule matches mail for any user at any host within our domain. It assigns the username to $1, the hostname to $2 , and any trailing text to $3 . The righthand side is then invoked to process these.
Let's now look at what we're expecting to see outputed. The righthand side of our example rewrite rule looks like: $#smtp $@ $2.$m. $: $1 < @ $2.$m. > $3 .
When the righthand side of our ruleset is processed, each of the metasymbols are interpreted and relevant substitutions are made.
The $# metasymbol causes this rule to resolve to a specific mailer, smtp in our case.
The $@ resolves the target host. In our example, the target host is specified as $2.$m. , which is the fully qualified domain name of the host on in our domain. The FQDN is constructed of the hostname component assigned to $2 from our lefthand side with our domain name (.$m. ) appended.
The $: metasymbol specifies the target user, which we again captured from the lefthand side and had stored in $1 .
We preserve the contents of the <> section, and any trailing text, using the data we collected from the lefthand side of the rule.
Since this rule resolves to a mailer, the message is forwarded to the mailer for delivery. In our example, the message would be forwarded to the destination host using the SMTP protocol.
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