Freshsales gives you the option to link email conversations to leads, contacts, and deals.
To link emails to deals,
Go to Profile icon > Settings.
Under the Email Settings tab, enable the option Link emails to deal.
The above configuration is a global setting. In order to associate an email when it is sent or received to all open deals of the email recipients, users can enable this checkbox. If you don't want a particular mail (like a confidential price quote) that you are sending to be listed under a deal's 'Recent conversations' section, you can prevent this from happening.
At the bottom of your email compose overlay, click the three dots icon and disable the checkbox Link conversation to Deal.
The global setting is the default setting and is reflected in the checkbox of the email compose overlay too.
You can enable or disable this as per your requirement.
You can view the conversations related to a deal in the Recent Conversations section in the details page of a deal.