using shared mail folders

Historically we've handled shared mail accounts by having multiple users 
know the password to one regular account. Now I'm looking at using 
IMAP native support for shared folders (on Linux).

I looked at #shared and shared regular accounts.
If I create imapshared with appropriate file permissions it appears in 
the Thunderbird subscription list. But if I want to have several shared 
accounts it seems I'd have to play games with mail delivery to get the 
mail in separate subfolders with different group memberships.
I see that dmail can deliver to e.g. +#shared/test

- question: if using the MIX format, what happens to the ownership and 
permissions of new folder fragments (whatever .mixnnn files are called) 
in shared folders ? Do they get set with mode 600 hence become unreadable 
by other group members ?



If I create a regular account with appropriate file permissions and group 
memberships I can access it easily in Alpine
as {mail.example.com/user=jane}~dick/INBOX
But in Thunderbird I can't see how to easily subscribe.
If I edit ~jane/.mailboxlist to include ~dick/INBOX then
I see the folder in the Thunderbird subscription list, but
as ~jane is not a shell account she can't do this herself.

I also get a complaint from sendmail that ~dick is group-writable, and 
sendmail refuses to read ~dick/.forward. Not perhaps a problem since 
forwarding a group account is unlikely.

Is anyone using these methods in production ?