Do you use Thunderbird's "adaptive junk mail controls" feature? That examines spam messages and "learns" to recognize spam. It's not infallible - it can make occasional false positives and negatives, but it usually works very well. It's possible, though, that it's training data may become corrupt and it might make more mistakes than usual. To test this, there are two settings you can use: Tools | Settings | Privacy & Security | Junk Temporarily disable the "When messages are marked as junk..." option; Tools | Account Settings | Junk Settings Temporarily disable the "Enable adaptive junk mail controls for this account". If that solves the issue and you want to keep using the Adaptive Junk controls, you can reset the training data. It will make mistakes for a while but the more spam it detects, the better at it it gets. Tools | Settings | Privacy & Security | Junk | Reset Training Data