From Garrison Computer Services:
Email Archiver is an AppleScript Studio application which sets up 'archive rules' for keeping the size of mailboxes to a reasonable limit. It allows you to specify which mailboxes you want to archive, how old messages must be to archive, and how often to roll over archive mailboxes. It currently works with Mailsmith, a version for Eudora and Apple Mail is underway. What is the advantage of archiving messages within your mail program rather than to text files or FileMaker? For some of my active mailboxes (eg. Clients, Projects, outgoing mail, etc) I want to keep all the messages, but I want the 'active' mailbox to be small and only contain current or recent messages. I used to manually move all older messages to date specific sub-mailboxes. After about 100 times I decided to automate the process; Email Archiver is the evolution of that original script.