PennyPost is a Thunderbird extensions that makes it possible to render spam obsolete. When the email you really care about includes a stamp with "proof-of-work", spam is easily disregarded. Every recipient in an email get a unique stamp valid for a short period, this makes sending spam very costly.

The extension needs to be configured immediately after installation before it can be used (Java is required).
The MBound algorithm requires a 16MB fixed table to work and is more memory intense.
HashCash is more CPU intense and is the algorithm of choice for Bitcoin