Over time, the file ruins the space in a significant amount, swelling and unnecessary. The only real solution for this is to create a new file without junk. It is called Compact folders in Thunderbird. People who use Outlook will be familiar with a signal to be compressed.
This is the same thing with a different name. When you delete or move e-mail from a folder, Thunderbird and other e-mail programs actually “hide” them, and these hidden E-Mail still live in the folder. Emptying the waste also does not remove them permanently.
These hidden e-mails remain in the folder until it becomes compact. If you do not compact folders, your mail folders can be very large. The program will also behave incorrectly, so it is a good idea to do this from time to time.
Do not confuse with compacting folders with compress file. This is not a unique feature of Thunderbird. The main difference is that by default most other email clients automatically compact the folder when some amount of space is wasted.
Compact Folders in Thunderbird
To compact all the folders in a single account:
Click on the account the left side and then click on “File >Compact Folders“. Depending on how much mail you have in compacting an account and how you have compacted the folders recently. it may take a few seconds or a few minutes. If you have a problem doing this and the process is trying to compact one folder at a time. click on the folder and select “Compact This Folder“.
Note: Try to avoid downloading e-mails while compacting the folder. It has been reported that it may cause corruption or “Folder is being processed” error.
For POP3 accounts: If offline support is installed, firstly go offline (“File > Offline> Work Offline”, or simply click on the icon that as look as a light bulb in the bottom left corner). For IMAP accounts: You can’t compact folders unless you are online.
Compact folders for IMAP accounts
For IMAP accounts, Thunderbird can automatically compact the inbox when you exit it. Go to “Tools > Account Settings > (Account Name) Server Settings” and exit on “Clear inbox”. If you check “Empty trash” at the exit time, then it will be empty and compact the Trash folder when Exit Thunderbird. You can also take help of Inbox IMAP setting for Mozilla Thunderbird.