We have implemented a shared IMAP folder in our mail server (running Cyrus-Imapd) where all users can move undetected spam to. This folder is used to train our spam filter.
The shared folder is configured in such a way that every user has list and insert access rights, meaning every user can see that the folder exists and has the right to copy mails into that folder. Notably, users do not have the r right, which would allow them to list all messages (including from other users who dropped mail in there), read the messages, search in the messages, and copy messages from the folder.
In Aqua Mail, I defined this folder to be our spam folder. But when I move a spam mail to this shared folder, Aqua Mail would complain with "Error synchronizing mail: Mail folder is missing on the server". Worse, it appears it would periodically try to move the mail to this folder, leading to a lot of duplicate copies in that folder (see attached screenshot) and leaving the original spam mail in the inbox.
K9/Kaiten doesn't exhibit this issue. To solve this issue, Aqua Mail could fine grain its permission checking on folders. In order to move a mail to a folder, a client doesn't need read access rights. The same applies to checking the existence of a folder. Just because a client doesn't have read access to a folder doesn't imply that the folder is missing.
Cheers
