Luckymonkey

Worked for mine - although the index size reduction isn’t massive (~24mb to ~20Mb), it’s the rebuilding of the databases indexing which makes it a good idea.

http://www.hawkwings.net/2007/03/01/a-faster-way-to-speed-up-mailapp/

It’s a shame that Mac Mail doesn’t do this automatically, since the load isn’t massive (would add a couple of seconds to load time once a week or something), but would save you easily more than that.

What it’s really doing can be explained here :

http://www.sqlite.org/lang_vacuum.html

In time inserts and deletes can leave the database file structure fragmented, which slows down disk access to the database contents. The VACUUM command cleans the main database by copying its contents to a temporary database file and reloading the original database file from the copy. This eliminates free pages, aligns table data to be contiguous, and otherwise cleans up the database file structure.