Junk mail

Thunderbird contains a highly competent spam blocker, which you can access via Tools —> Junk Mail Controls. The junk mail filter learns as it goes, so although it may mis-identify some e-mail initially, over time it will do a better and better job.

Make sure you enable the White List, which automatically approves e-mail from anyone in your address book. Also ensure you enable the Adaptive Junk Mail Detection (Tools —> Junk Mail Controls —> Adaptive Filter); this is the component which learns to detect spam over time.