The setting is in: Administration Center > Features > General.
Automatically predefine an active deal / project when sending a new email option gives you the opportunity to define whether the Superior Item of the saved sent email will be predefined with the superior item related to the email address unless the email address is related to more than one superior item (deal, or project). For more information, go to Automatic Email Saving.
Click Automatically predefine an active deal / project when sending a new email to turn the setting on/off.
If the function is turned on, the Superior Item is prefilled during the sending an email, if there is only one project, or one deal related to the email address of the receiver. You will see these deals and projects whose Contact Person or Customer have filled email address same as in email. You will also see deal that have filled same email address in their detail.
If the function is turned off, the Superior item field won't be prefilled.
When you change the settings, be sure to click Save. For more information, go to Features.