I am sorry, but how do you put it in learning mode? My understanding is each mail that comes into Thunderbird and each false positive you select as not junk and each mail you mark as junk all contribute to the filter learning at all times. In essence I am not aware of a special learning mode.

Normally when thunderbird is started for the first time or when you force relearn, it allows you to mark junk mail and starts marking but doesn't delete any on its own. I can't remember for sure but after a week or two it starts to actually delete emails mine never moves beyond marking them. Dusty