Did you set accounts in Thunderbird to be "mark as deleted" in account settings?

If not, then the fault is not Thunderbird.

Thanks for the reply.

Hmmm. Under Server Settings there's "Just mark as deleted" Under "when I delete a message." I can't find another such option. But the point is that I've used t-bird for years with this account and I haven't changed any settings lately. One day it was fine and the next day this happened. There was an update a few days ago and my Message synchronizing settings were changed to Keep messages in all folders for this account on this computer. I absolutely did not make this setting.

You have communicated what you are experiencing.

We need to better understand your environment. Let me ask this differently, under "when I delete a message." which of the three choices in selected? If is something other than "Just mark it as deleted", then I think the fault is not Thunderbird.

Also, who is the mail provider, and what antivirus software is installed?

You noted: "If is something other than "Just mark it as deleted", then I think the fault is not Thunderbird." This problem account, as well as the 2 accounts without the problem, are marked "When I delete a message: Move it to this folder." where the folder identified is the trash.

The problem account is comcast. My 2 other accounts are gmail. However I had the same problem with one of the gmail accounts about 6 months ago when it un-deleted about 10GB of emails. I keep Windows Defender on and also have the paid version of Bitdefender.

I appreciate your trying to help me with this.