I understand that when a new email arrives, Pipedrive look for that email in the data base, and if so, will link to a deal if there is "one" deal active.

But what happens when that person has multiple deals? How Pipedrive is working to "decide" to which deal it should link? Or in this case Pipedrive is not able to link the mail?

How Pipedrive works in this way will be very valuable to talk with our clients, since we really don't know. :)