"But 是连词it是主语 didn't take 是谓语very long 是宾语before they discovered that the most important thing was the ability to send mail around是状语从句,其中that从句是状语从句中的宾语从句, which they had not anticipated at all是非限制性定语从句修饰前面的the ability,"
本句意思:但是没过多久,他们就发现最重要的事情是能够四处发送邮件,这完全出乎他们的意料。