Multiple clients with the same referring agent's email address

My company receives referrals from outside agents. One outside agent will submit several clients, and their email is attached to each submission.

For example: John Doe may submit 5 leads to us. His email is associated with all 5 leads.
When I use Emails and Activities to update John Doe on a specific client, that same email shows on all the other clients that John Doe submitted. Is there a way to prevent this and the email only show on the client it was sent from?

You need to check if your email system supports individual client tagging to ensure correspondence appears only on relevant client records.

We use gmail - so I assume it can do individual client tagging. How do we set that up? Is it on the gmail side?