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Sendmail, 3rd Edition by Bryan Costales

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The localaddr Rule Set 5

For version 2 and higher configuration files V8 sendmail allows local recipients to undergo additional rewriting. Recall that each recipient address is processed by the canonify rule set 3 and parse rule set 0. Beginning with V8.7 sendmail, any delivery agent with the F=A flag set[5] (F=A) will cause the address to undergo aliasing (via the aliases file), which can result in a new local address.

Under V8 sendmail, if an address makes it through aliasing unchanged, it is given to the localaddr rule set 5, which can select a new delivery agent. Note that it is given to the localaddr rule set 5 after it is processed by the User Database (if used), but before it is processed by the ~/.forward file.

Beginning with V8.7 sendmail, any delivery agent that has the F=5 flag set (F=5) will cause the localaddr rule set 5 to be called as though the agent were a local one.

To illustrate, consider that a new delivery agent might be needed in the case of a mail firewall machine. A firewall machine is one that sits between the local network and the outside world and protects the local network from intrusion by outsiders. In such an arrangement it might be desirable for all incoming mail to be delivered to the firewall so that no outsider needs to know the real names of machines on the local network.

Consider mail to the address john@firewall. On the firewall machine, the parse rule set 0 selects the local delivery agent. Because the address john is local, it is looked up ...

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