See linked emails
You can see if an email in your Outlook inbox or outbox has been linked to a task. If you want to use this feature, you need to turn it on by checking the Add task properties to filed emails checkbox in the Tasklist ribbon tab.
The feature is optional because it adds two custom properties to the filed emails - don't use it if you don't want the properties added to your emails.
Once you have enabled the feature, drag an email on to your task list. Then right-click on the column header in your inbox or sent items folder, and select Field Chooser. In the field chooser, click the drop-down arrow next to Frequently-used fields, and scroll down to User-defined fields. Drag the Filed and Task headers on to the header bar of your inbox or sent items folder.
This will create two columns. If an email has been filed, the date it was filed will appear in the Filed column, and the task with which it was filed will appear in the Task column.
You can remove the columns by dragging the headers off the header bar.