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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?

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This is really more a development/programming question -- I know of no mailing list software offhand that requires this, but it could be hacked onto any of them easily enough (preprocess each message and ensure it has a valid PGP signature. Reject if not.) – voretaq7 Sep 12 '11 at 18:51
up vote 4 down vote accepted

Pick whatever MTA and list manager you want. Install procmail, write quick procmail recipe to look for PGP signatures - allow message if the sig is found, deny if not found.

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Probably the best generic answer possible without doing all the legwork for him. Courier-MTA also allows for scripts to run against submissions before SMTP/MSA accepts the transmission. – Chris S Sep 12 '11 at 19:00
No, this is not a perfect answer. But close to. The problem is to "deny" the mail after successful reception which causes bounce messages to be created (bad backscatter setup). Better would be to write a before-queue filter that does the same, but instead is able to reject during the SMTP dialog. – mailq Sep 12 '11 at 21:41

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