Trash or SPAM/Junk?

Either way I'd suggest checking if you have any filters that would trigger this, depending on the email provider some may have automatic Junk mail filtering which is sent to a folder that you don't see in Thunderbird.

See this web page for more information on subscribing to your email folders as provided by your email service.

I have removed all filters. The mail does not go to junk but to trash. I also use SpamAssassin

Check if your email provider has filtering enabled at the server level, perhaps some setting there is the reason for this.