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diff --git a/email2.md b/email2.md
index a1bcd2d..942f980 100644
--- a/email2.md
+++ b/email2.md
@@ -343,3 +343,7 @@ Links to more information, sources, etc.
   not have originated there.
   - <https://github.com/emacsen/rwot9-prague/blob/ap-unwanted-messages/topics-and-advance-readings/ap-unwanted-messages.md#postage>
   - <https://podbay.fm/podcast/252333772/e/1503662400>
+
+* Hashcash is a system where the sender proves they do some work
+  before the recipient accepts a message.
+  - http://hashcash.org/