Mail::Box

The disadvantage of delay-loading is that not all information is studied when you do not need it. Until now, I only discussed delay-loading of the body of a message which is common for Mbox-like folders. When you think about MH- and IMAP-folders, you will need delay-loading on the message-headers too! Even more delayed.

To find messages related in discussion-threads, you have to get the message-ids of the messages found in the folder. If you need all message-ids of an MH folder, you need to parse all files. This is unacceptable, and one of the reasons why many mail-user agents to not implement threaded-reading.

The trick of Mail::Box is to collect all message-ids from parsed headers. Then, when the thread of a certain message is requested, the folder is scanned from back to front (from most recent to the oldest), until

1. all referred message-ids are found, or
2. a certain threshold in time is reached (don't go further back than two days), or
3. a certain amount of messages is scanned.