[Dovecot] How to enable COPY and APPEND commands separately

[Alex Cherniak]

I'd like to allow a user to move messages between his folders on Dovecot
IMAP account, but prevent move/copy from different accounts (Exchange in
particular).
Outlook uses "xx UID COPY 1 folder" and then "xx UID STORE 1 +FLAGS
(\Deleted \Seen)" for internal moves and "xx APPEND folder" for external
ones.
I tried to achieve this with ACL, but i (insert) seems to control both.
Do I miss something? Should I look somewhere else?
Please help.