I could not figure how to attach an e-mail to a deal in the SalesInbox.

Our company is dealing with a lot of partners, and I will for example receive in the same day 5 e-mails from the same contacts but that should be associated with 5 different deals, how would I go about that?

At the moment, it seems that the only solution is to use the Outlook Plugin in Outlook to attach those e-mails.