最近在慢慢的學習 exchange 2013 的相關技術，在這個過程中看到的較重要的知識點希望能在這裏做一個記錄， 也能和大家一起來分享。
今天和大家分享的就就是 delayed fan-out 技術特點，
什麼是delayed fan-out?
我的理解就是，就是爲了解決當收件人多，且多數收件人在另一個站點，正常情況下發件服務器需要複製出和收件人數量相同的郵件副本，然後一一投遞給相應收件人。 然而 delayed fan-out 就很好的解決了此問題， 例如 一封10MB大小的郵件的收件人收100個， 其中有80個在B站點，那麼站點A 只需要向站點B 傳遞1份 郵件即可， 剩下的79份再由站點B 去複製， 這樣就能有效的減小AB 之間的流量，且能更快的投遞郵件. 這對於大型組織擁有多個站點的集團模型具有很大的優勢。
以下內容來自英文版教程：
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.