What is Mail.app reporting for errors or for diagnostics, or what are some of the unexpected behaviors you're encountering, when it "doesn't work"?
If you can get to this particular tool within Mail.app, what does the Connection Doctor report? (Mail.app > Window > Connection Doctor)
As for your question, there are two library folders around for each user. The first is the system-wide Library folder. The second is a per-user Library folder. Both folders are around and both are involved with Mail.app, though they're not visible (by default) in recent releases.
It's comparatively rare for a user to have to directly access either of these two Library folders — and changes made to these folders can cause problems, which is why Apple has hidden them.
What "patch" did you download, and from what site did you download it?