Compacting Thunderbird Folders

Ever wondered what exactly is done when you instruct Thunderbird or outlook express to compact folders. Here is a good article about this (or this one.)

Summary:
"When you delete or move e-mails from a folder, Thunderbird and other e-mail programs simply hide them (and mark them as ready for physical deletion later on), and these hidden e-mails still actually remain in the folder. Even emptying the Trash does not physically delete them. These hidden e-mails are physically removed only when the folder is compacted. If you don't compact your mail folders periodically then they can grow very large and erratic program behavior may occur.

E-mail users have often never heard of compacting folders (not to be confused with compressing files). However, this approach to email storage and deletion is common to most e-mail clients. Its a way to improve performance by avoiding rewriting the entire file every time you delete a single message. The difference is that most other email clients by default automatically compact the folder when a certain amount of space is wasted; Thunderbird does not do this."