If this is an IMAP account, you might check with your email provider, as they may have a policy on duration of trash.

Do you access this account from another device such as a phone? Check the settings for server behavior there.

Thanks for your replies.

David: I checked with my email provider and they said there is no limit on the size of the trash folder, so seems like that's not the problem.

Rick: I access email from both the Thunderbird desktop client and the Thunderbird app on my Android phone. I'm not seeing any obvious settings that would limit the size of the trash folder; do you have any suggestions? There is an account setting on my phone (Account settings/Folders/Trash folder) that can be changed from Automatic (Trash) to Trash...would that make a difference? I've added a screenshot.

One thing I've noticed is that the first time I opened the Trash folder today, I could see the number of messages at the top go down immediately (e.g., from 248 to 245).

My guess is that either your phone or your webmail is set to delete messages from the trash folder after a month and that you need to find those settings.

Do you use message filters in Thunderbird?

No, that setting on your phone is for where deleted messages go, not for how long they stay there.

Thanks, Rick. I'll continue to try and find any delete settings.

I don't use message filters.

Thanks for your help