When you delete emails they are marked as deleted and hidden from view, but in reality they are still in the file. This makes it possible to recover accidentally deleted emails. Compacting removes those 'marked as deleted' emails and recovers space. The Inbox should not have too many emails. It is best to move wanted emails into other suitable folders for storage. It is also advisable to compact Inbox and Junk/Spam folders on a regular basis as these two folders get the most activity especially when it comes to deleteing. I delete quite a few emails each day and therefore will compact those two folders each day or every other day. Compacting does not delete emails, it removes already marked as deleted emails. Some people have experienced a loss of emails after compacting. This occurred because the file was already corrupted. Corruption can occur if you do not maintain a healthy Inbox and compact on a regular basis or if an AV product has scanned it and caused an issue in the file. If Thunderbird cannot determine which is the start and end of a 'marked as deleted' email, the file is corrupted it can remove good emails. If you suspect a file is corrupted or are concerned in any way because you have not compacted on a regular basis then do not compact the folder until you have followed the instructions below. If in doubt or concerned, do the following. Move all wanted emails out of Inbox into other suitable folders. Delete any emails you do not want. When Inbox is empty.... Right click on 'Junk/Spam' folder and select 'Empty Junk' Right click on 'Junk/Spam' folder and select 'Compact' Right click on 'Inbox' folder and select 'Compact'