Just a quick update on my FastMail Configuration post: After I wrote that post I ran into some sync issues. Mail.app on OS X had massive problems listing the contents of my folders (and even listing those folders themselves), especially with folders located directly within
I had a lengthy conversation with Yassar from the FastMail support team about this issue. We finally figured out that it seems to a bug in Mail.app and he later told me, that they tried to work around this for quite some time. He showed me some posts (1, 2) on their blog regarding similar issues going back as far as 2008.
Long story short: What I ended up doing was moving my
INBOX subfolders one level up (so that they now live on the same level
INBOX does) and left the rest of the configuration as is. Works for me™.