First, I suggest checking online to see if your email provider is placing in spam before message is seen by Thunderbird. That is a common situation, as many email providers do basic spam checking. Next, I suggest creating a filter to move the message to inbox. The junk filter needs ongoing maintenance and teaching to improve results. This URL has additional information: https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages But for one important email id, a filter can be a good choice.