Google-friendly title, no?
In your ~/.procmailrc
MAILDIR=$HOME/Maildir – is BAD, whereas
MAILDIR=$HOME/Maildir/ – is good.
The slash at the end appears to tell procmail that this is a Maildir directory, rather than something else, elusive.
For a week my messages were appearing in $HOME/Maildir, but they were just in there with names like msg.zFS, instead of going into $HOME/Maildir/new and having a name like 1282544959.Q22432E220.127.116.11.2:1,T.
This meant they effectively disappeared, and I had a quiet, relaxing week.
Followed by a panicked and expletive-filled hour or so.
When I put the slash back, new messages started appearing in my Inbox again.
I was also able just to move the msg.zFS files into “new”, and they appeared fine, which was nice.