If you exit thunderbird each day and you have a setting to empty the trash on exit, then this may explain what is happening.

• Right click on mail account in Folder Pane and select: 'Settings'
• Select : 'Server Settings' for the account

Under 'Message Storage' section..

• uncheck 'Empty Deleted folder on Exit'
• click on 'OK'

Another possibility is the 'Retention Policy' settings. This can be set in Account Settings or on that folder.

Check Account Settings:

• Right click on mail account in Folder Pane and select: 'Settings'

If this is a POP mail account:

• Select 'Disc Space' for the account.

If this is an Imap mail account:

• Select: 'Synchronisation & Storage'

Under the 'Disc Space' section

• Select 'Don't Delete any messages'
• Select checkbox 'Alwaya keep starred messages'
• click on 'OK'

See if the folder has a different Retention Policy to Account Settings:

• Right click on the Trash/Deleted folder and select 'Properties'
• Select the 'Retention Policy' tab
• Select checkbox 'Use my account Settings'
• click on 'OK'

Toad-Hall said

If you exit thunderbird each day and you have a setting to empty the trash on exit, then this may explain what is happening.

• Right click on mail account in Folder Pane and select: 'Settings'
• Select : 'Server Settings' for the account

Under 'Message Storage' section..

• uncheck 'Empty Deleted folder on Exit'
• click on 'OK'

Many thanks - so simple when you know!