最近在慢慢的学习 exchange 2013 的相关技术，在这个过程中看到的较重要的知识点希望能在这里做一个记录， 也能和大家一起来分享。
今天和大家分享的就就是 delayed fan-out 技术特点，
什么是delayed fan-out?
我的理解就是，就是为了解决当收件人多，且多数收件人在另一个站点，正常情况下发件服务器需要复制出和收件人数量相同的邮件副本，然后一一投递给相应收件人。 然而 delayed fan-out 就很好的解决了此问题， 例如 一封10MB大小的邮件的收件人收100个， 其中有80个在B站点，那么站点A 只需要向站点B 传递1份 邮件即可， 剩下的79份再由站点B 去复制， 这样就能有效的减小AB 之间的流量，且能更快的投递邮件. 这对于大型组织拥有多个站点的集团模型具有很大的优势。
以下内容来自英文版教程：
Delayed fan-out
Exchange uses a technique called delayed fan-out to optimize the use of network bandwidth to transport messages. After a message goes through the categorizer, Exchange
knows the full recipient list and can then calculate the routing path for the message. After
you have a message with multiple recipients, it’s likely that different routing paths are
necessary to get the message to the destination servers. Typically, email systems fan out
and create as many copies of the message as required to travel the different routes. This
technique works, but it means that all the copies are created on the originating server, and
each copy must be processed separately. If some of the recipient mailboxes are in the same
database, multiple copies of the same message travel across the same connector to the
same destination.
To avoid this problem, Exchange examines the routing path for each recipient of a message
to determine how it can transfer the fewest copies of the message across common routing
paths before it needs to fan out into separate copies. Some copies will have to be generated immediately, but in many instances, a single copy can be routed across a common link to another Mailbox server in another site, which can then create multiple copies for local
delivery. The determination of the most efficient delayed fan-out for a message is arrived
at by identifying the hub transport servers that will create multiple copies. Each of these
Mailbox servers is referred to as a fork in the routing path.
