The Cyrus IMAP server

The Cyrus IMAP server was developed by Carnegie Mellon University as a replacement for their Andrew Message System. Cyrus has a number of interesting properties as an IMAP server:

One message per file

• Minimal overhead removing messages from anywhere in folder.
• Supports single instance store layered on top of a normal filesystem.

Index and Cache files.

• No expensive stat() operations required when opening mailfolder (other file per message stores can work around this one by building information into filenames).
• Don't have to open() every message for threading or searching
• Can return BODYSTRUCTURE and individual MIME attachments without having to parse message

Safe concurrent access:

• Allows simultaneous access by same user at different locations
• Each user has their own independent record of messages seen in a folder.
• Full ACL system: Much finer grained than Unix permissions.

Local Customisation to Cyrus

• Compatibility
• Performance Enhancements
• A two phase expunge commit system so users can recover from accidents
• Replication for data migration and disaster recovery

The patches are available here.

We also wrote a small, stateless, IMAP and POP proxy server. This was principally because the Murder software was still rather immature when we started this project in 2002 : it seems to have stabilised quite nicely in the intervening time. The Murder has the substantial advantage that it provides transparent support for shared mailfolders.