I'm not clear on what you mean that you can't select from the dropdown. Do you mean the options are there but you cannot access the options - or that you don't see results?
It works for me. You should be able to select from from, to and all in that dropdown. However, the options will appear in the dropdown, whether there is mail or not.
You're probably already doing this, but to confirm:
If there is more than person with that name / search entry, you will need to select an entry under People first.
The to, from, people dropdown works fine for me - if there is mail in the category. If none apply, you might have no dropdown.