I found an article which is nearly correct. Here is my correct solution:

Turn off and disable ALL junk filtering

1. Go to account name at upper left, above Inbox and below “+New Message”. 2. Right click on that 3.Go to “Settings” which is the last entry in the drop down menu. 4. Go to “Junk Settings” 5. UNCHECK “Enable adaptive junk mail controls for this account” 6. Go back later and confirm that the box is still uncecked. 7. Go further down to “Junk Settings” in “Local Folders” and do the same thing (usually this is the same screen as before and usually will be unchecked already).

The article at https://support.mozilla.org/en-US/questions/1061256#answer-726506 has nearly correct instructions.

DOES NOT WORK! I did the above. Important emails from one email address are put in "Trash" instead of my inbox. To access these messages, I must go to the "Trash" folder. To save these messages for future reference, I must move them to the Inbox. Thunderbird is USELESS. A terrible program. Unfortunately, it is the only local email client available. Whoever created Thunderbird outsmarted themselves. This program has so many bells and whistles that it is extremely difficult to use.

If you have disabled Thunderbird's Adaptive Junk Mail Controls as you describe, it will not mark messages as Spam. And unless you specifically set it up to do so (usually using Message Filters,) it will never put New messages directly into the Trash folder. You would have to have a very good, very unusual reason to do that.

Perhaps you meant to type "Spam folder" rather then "Trash folder" above? If so, it's likely that your email service has its own spam/junk mail screening feature and is moving it before Thunderbird ever sees it. That could be checked by going to web mail before using Thunderbird. If that feature can be controlled by you the user, it would be done at the web mail page of your email service.

There are many local email clients besides Thunderbird.

The messages at issue appear in the TRASH folder and not in any other folder.

The email client I used before Thunderbird was Juno. I changed to Thunderbird because Juno stopped working in July of 2025. Thunderbird enabled me to keep using my Juno email address --- a good feature of Thunderbird.

Juno does not filter spam or junk.

What other email clients are there besides Thunderbird?

My small experience with a friend who uses Juno, and a quick google search tells me that Juno only offers web mail, not a local client. Maybe they did in the past. In any case, my search found that Juno does, indeed, have some server-side spam filtering and there are some basic controls available through the web mail interface.

Off the top of my head, some desktop email clients I'm aware of are:

Claws Outlook Blue Mail Mailspring Betterbird Spark Desktop eM Client

A web search will find more.