you might try changing the amount of disk space to be saved that triggers the compact function. That is occurs is a necessary evil. How frequent is a function of how much disk space it should same.

Options > advanced > Network and disk space. Perhaps set it to 100mb or so. Note that having lots of deleted mail setting in a folder which occurs when you do not compact increases the risk of false positives from anti virus and subsequent data loss from drastic attempts o remove the virus.

In addition to good info supplied by Matt regarding the auto compact feature.............

General info: Emails are stored in mbox text files, one after the other in the order downloaded. So they are not stored in individual files. Whilst you see the 'Inbox' as a folder in the Folder Pane, in reality those emails are stored in a file; the Inbox is a single text document.

When you delete an email, it is not fully deleted. It is hidden and 'marked as deleted'. This means it is possible to retrieve emails that are accidentally deleted. When you compact a folder, those hidden marked as deleted emails are fully removed.

This cleans up the file, reduces it's size and helps to maintain the file. Your question relates to the auto compact feature which was covered by Matt.

You can manually compact a folder at any time:

• Right click on a folder and select 'compact'.

You can set up Imap accounts to automatically 'Clean up (Expunge) Inbox on Exit'.

• Right click on imap mail account in Folder Pane and select 'Settings'
• select 'Server Settings'
• select checkbox: 'Clean up (Expunge) Inbox on Exit'
• click on 'OK'

Info on Compacting: http://kb.mozillazine.org/Thunderbird_:_Tips_:_Compacting_Folders

Firefoxor,

Does your issue continue in version 68?