rsync failing to copy files in a maildir

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http://serverfault.com – I'm using rsync --delete -Rav remote:/path/to/maildir/cur /backup/ but I'm finding lots of files on remote that are not getting copied by rsync, while others are copied. Rsync does not throw any errors, it's like it just doesn't see some files. I've checked permissions (remote is set up in ssh.config). An example file that is not copied is named 1343249107.H476717P21185.mail.example.com:2,S I created a copy of this (cp -a 134... copy) and this new copy file was copied by rsync. As you can see from m y command, I'm not using any --exclude options. What could cause rsync to fail like (HowTos)