How about right click the folder and select empty trash.

You might want to right click and select properties then the repair button as it sounds like either the trash folder or it's index is corrupt (1/1/70 dates) Repair will fix the index, but a compact will be needed to fix the actual folder data.

Note that a common cause of such things are system activities that interrupt the file access and write. Things like antivirus scanning, cloud syncing of the location, slow network responses when the data is on a local server or NAS or some form of streaming backup is in use.

Thanks for the reply. I don't want to delete all the trash. I want to keep the last few months. Is that possible?

Are you talking about a gmail pop account?

It is quite normal for many servers to auto empty the server Trash folder periodically and that can range from a few days to a month. But this would only effect people using Imap mail accounts.

If you are using a gmail pop account, it could mean you are not deleting emails off the server if emails are not put into the server Trash folder. Advise is to periodically logon to webmail account and check to see if really old emails need to deleted in order to increase the amount of quota you have on server.

How to delete messages in Trash that are eg: older than 30 days.

• Right click on Trash folder and select 'Properties'
• Select the 'Retention Policy' tab
• Uncheck the checkbox 'Use my account settings'
• Select 'Delete messages older than' and choose the number of days eg: 30
• click on 'OK'

The problem was not with gmail. Regrettably the problem has fixed itself when I accidentally deleted the trash in Thunderbird. Not a catastrophic issue as all the deleted emails are still in Gmail