Postfix 2.7 将引入动态选择源IP的功能

Major changes with snapshot 20100117
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The FILTER action in access maps or header/body_checks now supports sender reputation schemes that dynamically choose the SMTP source IP address.

This is implemented by specifying FILTER actions with empty next-hop destinations in access maps or header/body_checks, and by configuring in master.cf one Postfix SMTP client for each SMTP source IP address, where each client has its own "-o myhostname" and "-o smtp_bind_address" settings.

这样，可以让信用更好的用户通过一个可信度高的 IP 发送出去，而其它的用户，则走另外的 IP