The IMAP module supports RFC2086 (IMAP4 ACL extension). This protocol extension allows IMAP users to grant access to their Mailboxes to other users.
See the Mailboxes section for the detailed description of Mailbox ACLs.
In order to set Access Rights, a client should use a decent IMAP client that supports the ACL protocol extension. If such a client is not available, Mailbox access rights can be set using the WebUser Interface.