Q: What special kind of IMAP mailbox is "foo@user.alice."? Preparing an IMAP server migration and checking for connected components of users sharing mailboxes, we stumbled over a some unusual ACL patterns observed when running listaclmailbox * in cyradm (server is running Cyrus IMAP 2.5.6):
foo@user.alice.:
  anyone lrs

Obviously, alice shared something readable to any user logged in. We're unused to this syntax: usually, we expected something like
user.alice.foo:
  alice lrswipkxtecda
  bob lrs

when alice shared her mailbox user.alice.foo with bob. What is the @ indicating, why is there a trailing dot .?

A: I think you have a folder user.alice.!foo in mailboxes.db, and either there's a bug in 2.5.x, or you switched on virtual domains at some point.
The '!' character is used as the domain separator, which makes the folder name ambiguous.
This folder is probably invisible to user alice right now.
