[Speedslug]

If you can look up the file details it should have a date it was last modified that can tell you when you downloaded it on to your machine.
That might help you find the email the original password key came with.
You can sort the file lists by clicking the date column header.

Finding out where your machine stores the email files is going to be another hunt though. Look for the email settings menu for that.

Try searching Google for that exact title as well. Using "quotation marks" will tell Google for that specific phrase.