Mailing list + PGP/GPG support

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http://serverfault.com – I would like to create such mailing list where every contributor must sign his/her messages with PGP/GPG, otherwise mailing list software would block unsigned message. All PGP/GPG public keys of users allowed to post should be stored somewhere on mailing list server. All other users are allowed to read mailing list in read-only access at the same time. To what open-source mailing list software this feature can be easily added? (HowTos)