what is your mail providers retention policy on mail over 30 days old? IMAP is synchronized so their policy is relevant as you are questioning where your mail went.

Many years of emails are still available in my online Gmail account. I'm unable to find any settings changes in Gmail that would have caused this recent change within Thunderbird. And to further clarify, I move emails once read into "Local Folders" within Thunderbird. And only recently have emails older than 30-days not been available within these sub-folders.

re: Many years of emails are still available in my online Gmail account. - I move emails once read into "Local Folders" within Thunderbird.

If you were using an imap account and 'moved' (not copied) mails from imap folder into 'Local Folders' account, then the 'moved' email would get deleted from the imap account and also server. It should not be visible in webmail. BUT, are you saying they are still visible in the webmail account ?

Do you use a POP mail account for the gmail account in Thunderbird? This would explain why the webmail account would look different to the pop account, although usually you would move emails from pop Inbox into another folder within the same pop acount. Pop accounts store downloaded mail locally on computer - the same as the 'Local Folders' mail account, so no real need to move them.

re :I move emails once read into "Local Folders" within Thunderbird. And only recently have emails older than 30-days not been available within these sub-folders.

So, the effected sub folders are in the 'Local Folders' mail account - not the gmail pop or imap account. Meaning you need to check the settings for the 'Local Folders' mail account not the gmail account.

• Right click on 'Local Folders' mail acount name in Folder Pane and select 'Settings'
• For 'Local Folders' mail account - Select 'Disk Space'
• What is selected ?
• Suggest you select 'Don't delete any messages'

Also check the view. Select one of those sub folders. Check the 'Quick Filter Bar' - does it have any of it's icons highlighted eg: pin or starred etc causing the view to be filtered?

In Folder Pane: Check the properties of the sub folders under 'Local Folders' account

• Right click on those Local Folder sub folders and select 'Properties'.
• Select 'Retention Policy' tab and check settings.

It should have the following selected: 'Use my account settings' and 'Don't delete any messages'

Also check to see if the index is out of sync:

• Right click on those Local Folder sub folders and select 'Properties'.
• click on 'Repair Folder'
• click on 'OK'

Please confirm: Are you using a POP or an IMAP account for the gmail account in Thunderbird?

I am using IMAP within Gmail. Yes, I am 'moving' emails from the Inbox to the Local Folders; and 'yes' this removes them from the Gmail Inbox too. Within Gmail, I can see all the old messages in "All Mail".

As you suggested, I went to 'Disk Space' in 'Settings' for the 'Local Folders' and the selections are 'Don't delete any messages' and 'Always keep starred messages'

Investigated the 'Quick Filter Bar' and there's nothing selected.

Investigated the 'Retention Policy' under 'Properties' for one of the Sub-Folders under the Local Folders ... the 'Use my account settings' box is checked plus the 'Don't delete any messages' radio button' is checked ... all others grayed out.

And, I tried using the 'Repair Folder' button under 'Properties' for one of the Sub-Folders where everything but the last 30-days of email is missing. No update or repair obvious.

Thanks for all the details and suggestions! Perhaps there's a way to recapture 'All Mail' held within the Gmail account and see if only current comes over, or if older emails disappear on shut-down and restart.