Let's start here:

Go to Tools > Message Filters

Choose the account you're using

Look for any filters that have “Move to Trash” as an action

If found, disable or delete them

WHo is your mail provider? What version are you running?

thorsak, it's hard to help you when you don't reply.

Also, please use your original topic for this issue at https://support.mozilla.org/en-US/questions/1514822.

Please bookmark https://support.mozilla.org/en-US/user/thorsak/questions to find your past topics. Use this link in the future to keep track of them.