Move it to your Inbox in Local Folders then mark it as Not Junk. If it stays good, then it is your email provider's server doing the damage, despite appearances. Go to their website to see if they allow you to exert any influence over the workings of their spam classifier. Thunderbird's Junk Controls pay no attention to the sender, as this is invariably spoofed (falsified) in spam messages. You might be able to reduce the number of times this happens with a Thunderbird filter that moves the message to a safe folder, but it's always going to be a race between Thunderbird and the email server and so can't be guaranteed to always work. You need to fix this at source, i.e. on your provider's server. A filter running on the server might be more effective than those in Thunderbird that can only be applied after download. But the real fix it to whitelist this sender at the server and effectively prevent the misclassification.