# Courier-IMAP version 4.0

Contents of this NEWS file:

> Introduction
> Shared folders and access control lists

### Introduction

Courier-IMAP 4.0 brings many changes and improvements. All authentication-related code in Courier-IMAP has been moved into the external Courier Authentication Library. See the contents of the INSTALL file for more information. Courier-IMAP now includes an IMAP/POP3 aggregation proxy, like Perdition. See the "imap/README.proxy" file in the source distribution for more information. Mail accounts can be distributed across a pool of servers. A separate server (or another pool of servers) accept connections from IMAP and POP3 clients, waits for the client's login request, looks the mail account's server, and connects to the IMAP/POP3 server on the right host, with the IMAP/POP3 client being no wiser. Any kind of an upper limit on the scalability of the IMAP/POP3 server, or how large of a mail store it can support, has been eliminated. The only limit to Courier-IMAP's horizontal scaling is the available bandwidth on the LAN.

### Shared folders and access control lists

New to Courier-IMAP 3 are the ACL (access control list) IMAP extension, and ACL-based virtual shared folders. Courier-IMAP 3 implements both the existing ACL specification, RFC 2086, as well as an experimental implementation of the second draft revision of the ACL specification. Virtual shared folders is a different implementation of shared folders that supplements an older implementation in Courier-IMAP 2, and earlier. The older, POSIX permission-based shared folders are still implemented and supported. Here's a summary of the differences between the two shared folder mechanisms:

> POSIX-based shared folders | ACL shared folders
> --- | ---
> May be used with system accounts only1 | May be used with virtual accounts only; cannot be used on systems that have end-user login shell access: bypassing ACLs and accessing the folders directly is trivial
> Access rights based on traditional POSIX filesystem permissions | Access rights based on IMAP access control lists
> Sharable folders must be set up manually, using the "maildirmake" command | A site-specific process must be installed to compile an index of all virtual mailboxes; afterwords, individual users may create and manage shared folders themselves

> > 1 This is because access rights are governed by filesystem permissions. POSIX shared folders may also be used with virtual accounts, but it will not be possible to grant read-only access to shared folders, and everyone will be able to delete messages from shared folders (instead of only the creator of each message)

After installation, read the "maildiracl" manual page for a brief overview of access control lists.

Read "maildir/README.sharedfolders.html" for information on configuring virtual shared folders