When you receive an email, in order for it to show up in the person's timeline, you need to add the sender as a practice contact.
Expand the email and click on the sender name. This will display a preview card with the name. If it reads Add contact, it means the contact is not part of the practice, so click on it.
Once you do, you can select whether you want to create a new contact or add this one to an existing one.
Tip: If you select "add to existing" and the contact doesn't exist, you'll still be able to add the new contact.