Before your emails will start automatically showing against contacts you need to start sending them to your "Unique bcc email address". There are 2 methods to achieving this;

Method 1: Manually use your unique bcc email address

When sending an email to a client you can simply bcc the email to unique bcc email address. 

Method 2: Setup a bcc rule in your mail program

Most good mail programs will allow you to create rules that automatically do things to your emails, you can use this functionality to automatically bcc every email that you send to unique bcc email address. We have compiled a few tutorials (written by third parties) below for some of the major mail programs;