That depends if the spam detection is being done by Thunderbird, or if the mail provider (hotmail) is doing it. In thunderbird's account settings under junk you can set the address books which will be considered as a white list for junk mail processing. If the sending address is in the address book and it is selected for white listing, then Thunderbird will not classify mail from that address as junk. What the folk over at Microsoft do with hotmail I have no idea. I gave up trying to understand their machinations years ago.