长难句

Email’s privacy model was always based on courtesy: We wouldn’t look at the messages crossing the network that weren’t for us because that would be rude. It would be even more rude to change them, though system administrators did regularly insert strange messages or modify messages as jokes, or to get their users’ attention. Emails from God or Santa Claus were not unheard of.

句子切分

第二句：

We wouldn’t look at the messages crossing the network that weren’t for us because that would be rude.

主干识别：

We wouldn’t look at the messages　我们就不会看这些讯息

切分成分：

1.crossing the network　修饰messages的-ing形式；

什么样的讯息，在网络中穿梭的讯息

2. that weren’t for us　定语从句，进一步修饰messages，

什么样的讯息，不是给我们的讯息

3.because that would be rude　原因状语从句；因为看了很失礼

第三句：

It would be even more rude to change them, though system administrators did regularly insert strange messages or modify messages as jokes, or to get their users’ attention. 

主干识别：

It would be even more rude to change them　改动它们就更无礼了

切分成分：

1.though system administrators did regularly insert strange messages or modify messages …虽然系统管理员时常植入奇怪的消息或修改消息……

Did 在这里表示强调

2.as jokes, or to get their users’ attention 为了开个玩笑或吸引关注

词汇突破

1. privacy 隐私；保密

2. model 模式

3. courtesy 礼节

4.　cross 横贯；穿行

5.　rude 粗鲁的

6. regularly 定期地

7. insert  插入

8. modify 修改

9. Santa Claus 圣诞老人

10. unheard of 闻所未闻 

not unheard of：都听说过

参考译文

邮件的保密模式过去一直以礼节为基础：那些网络中穿梭的讯息，不是给我们的我们就不会看，因为看了很失礼。改动它们就更无礼了。虽然系统管理员时常为了开个玩笑或吸引关注植入奇怪的消息或修改消息。不是没有听过来自上帝或圣诞老人的邮件。（这就是管理者开的玩笑。）