Click the start next to the sender's address in the Header Pane to add them to Personal Address Book (click twice to choose another address book), then check the address book in Junk Settings so the sender is never marked as junk. But if mail is ending up in a Spam or Bulk Mail folder, that is done by the mail service before messages reach TB, so add the sender to webmail contacts or whitelist them according to the options provided by that service.

It's an old post but I have this problem too. I'm not sure what you mean by " then 'check the address book in Junk Settings' so the sender is never marked as junk" Where do I find the junk settings? Thanks

In Tools/Account Settings (see picture).

Duh, of course ... thanks!!