HASHBL blocklists store blocklisted data as one-way cryptographic hashes (tokens) rather than as unencrypted strings. Otherwise, they work in the same way as IP or domain-based blocklists; they provide a list that mailservers or spam filters consult to determine whether to accept, filter, or reject inbound email.

Cryptographic hashes are a useful method of listing data in blocklists for the following reasons:

The first MSBL project, the Email Blocklist (EBL), is currently in private, first-stage beta testing. Support for EBL and the HASHBL platform is currently under development on several spam filtering platforms. Once E-Drop goes into second stage public beta, test versions will be available from this website.