I'm using Spotlight and the folder is in Mail. Assume the folder name is "A Big Mystery" and I conduct a search just using "Big".
I'm just trying to understand your thinking here. The folder is a mail folder as seen in the left column of your mail app?
First, I'm not sure why one would be searching for that in the first place, but anyway... if it is that type of folder, it's not actually a folder in the finder. It's a .mbox which is recognized as a "Mail Mailbox" kind rather than a "Folder". Also since mail is stored in your Library, the Spotlight search will not find items there in raw file terms, but only in message metadata form.
If I Find in the Finder and include my system files, I get the .mbox files in my results when searching for my Mail "Folders".
If that's not the type of folder you're searching for, I think it needs to be clarified further, because I'm having trouble determining what a Folder in Mail would be otherwise.
