Recently when I was working on one of the project requirement, we wanted to enable Custom Entity to be available while selecting records in Email Activity party field To, CC and BCC.
To achieve this requirement we have to enable the custom entity metadata settings.
Step – 1 Navigate to Settings->Customization->Customize the system
Step – 2 Expand Entity ->Select Custom Entity
Step – 3 Tick Sending Email checkbox
Note – If an email field does not exist, one will be created †
Step – 4 Save the changes and publish.
Now once you publish the customization and navigate to Email entity record you should see currently enabled custom entity record visible to be selected in Activity Party field.
Hope this helps!