Address To Be Checked Indicator
Although you might have the address for someone, you might find that you are unsure if it is still the correct address. Perhaps you haven’t had a reply from that address, for quite a while.
If so, you can indicate that the address needs to be checked. To do so, Control-click (right-click) on the name and select Flag Address to be Checked.
Once you do this, a question-mark will appear after the address.