Are these messages marked as 'unwanted' by TB or by the mail provider, before they are downloaded to TB? You might want to whitelist the sender through the webmail access, or add the sender to the webmail contacts.

Thank you very much for your answer! I thought TB is my mail provider, but in the mean time I discovered it 's probably outlook.com. I am not a computer expert. Now I need to figure out how to prevent the mail provider from marking messages as junk. I will keep you informed.

Thunderbird is an email client. It gets emails from a mail providers mail server.

Outlook.com (Microsoft) is your mail provider. How you use their web site and configure their settings is not something anyone here is really competent to tell you. Microsoft offer their own support for Outlook.com. I am certainly not an expert in their web site. But this link "should" take you to the safe senders and domains settings. https://outlook.live.com/mail/0/options/mail/junkEmail/safeSendersDomainsV2

If it does not work, search the outlook.com help for safe senders.

Thank you, Matt. It worked!