Sorry for late reply, I wanted to test things first. I installed Thunderbird again, IMAP style. My email provider is altiboxmail.no. When I send mail directly from their internet mail page, it doesn't end up in spam.
There was a meeting and I sent out seventy emails, and one person told me it arrived in her spam box. We have a facebook group too. I hope that not more people got it in their spam box, but couldn't be bothered to tell me. Her email is hotmail.no. Nobody else had a hotmail.no account, so I guess it could be that.
I sent an email to that test site, and it came back with a rate of 7/10, that they said was good.
I don't actually have a spambox, because I get so little spam, since I never give that email to web pages. I did it once last year for an email newsletter about health, whereafter I got lots of health spam, and after a few months of clicking unsubscribe I actually emailed and even called the companies that the commercials came from and gave them a lot of swear words for not stopping the spam, and it stopped, except that one of them have sold my address to some phishing people.
Thank you all for taking the time to answer. Because of that one person I still don't trust Thunderbird, but I guess I can use the internet interface to mail if I am sending to someone new/important.