Mail Activity Filter Internal User

This module changes the small popup window used to schedule an activity (the one with the summary, due date and assigned person).

• A new checkbox labelled "Assign to internal user only" appears just above the "Assigned to" field. It is ticked by default.
• While the checkbox is ticked, the list of people you can pick from only contains internal users; users who are portal or shared users are hidden.
• If you untick the checkbox, the full list of users becomes available again, so you can assign the activity to an external or portal user.
• The checkbox is a setting on the activity itself, so it is remembered on that activity record rather than being a global preference.

The module extends the existing activity model mail.activity and only overrides the standard activity scheduling popup view. It does not add new menus, reports, emails or scheduled jobs, and it does not change how activities are completed or how reminders are sent. Its only effect is to make the default choice of assignee safer and quicker for day-to-day internal work.