Received emails will be automatically linked the the contact who sent you the email. However, you can also link the email to a project and other contacts by simply updating the Project and or Related to fields on the email. The email will then be visible under any of the linked records´s Activity section.
Link a Project – on a received email, tap Project and then select a project.
Link Contacts – on a received email, tap Related to and then select one or more contacts.