Thunderbird
Thunderbird is a popular e-mail client. There are some technical bits of information here about that client.
Compacting Folders
If you use Thunderbird as an e-mail client, then you should be aware of the growing file size issue in Thunderbird.
Over time, your Thunderbird folder files (like Inbox) will continue to grow even if you delete mail in them. This is because the e-mail is not really deleted inside the file. It is only marked then not displayed on your screen. This is a programmer's trick to make programs run faster by not making the program erase and rewrite the database file every time you delete a mail.
There is a feature called "compact" which will remove the deleted files from the database. However, Thunderbird will not compact e-mail boxes (like Inboxes) unless you switch the feature on in the Tools -> Options -> Advanced -> Network & Disk Space -> Disk Space menu. I recommend that you set the "Compact folder when it will save over" to 1000 KB (which is a 1 MB). This feature is set OFF by default in Thunderbird.
You can also compact the folders manually. Manually compact folders by selecting the account in the All Folders window, then choose File -> Compact Folders. As an example, my Inbox folder file was compacted from 1 Gigabyte down to 37 Megabytes. The file size is now 3.7% of the original file size.
Managing and Restoring a Thunderbird Profile
See this link for details on restoring a corrupted profile:
