The information contained in this e-mail and any attachments are strictly confidential
分析一下这句话
首先找句子的最关键--谓语动词,本句里是are,be动词.
其次,主语,是the information
句子主干就是 The information are strictly confidential.
而contained in this e-mail and any attachments(包含在该邮件和任何附件里的) 则是做了主语information的后置定语,还原应该为 the information which/that is contained in this e-mail and any attachments(定语从句).