Without any agreement of the writer to the contrary — this is an explicit violation of the writer’s Copyright Act Section 106 exclusive rights.

Minus a subscription agreement — we can argue on the basis of copyright law that there is an “implied license” for a mailing list post to be posted to the list, because the author has sent it to the mailing list address with the intention that it be distributed to all members of the list.

However, it is not the case that an author of a mailing list post intends to give anyone and everyone free rein to reproduce the post. Giving informal permission for one copyright use, does not give permission for any use of a copyrighted work.

Further, many mailing lists are private or subject to restricted access. Businesses and professional groups use mailing lists as a type of “virtual meeting” which allows the participants the convenience of not having to be in the same place to have an ongoing discussion. In order to encourage frank discussion and full participation, and sometimes to protect professional reputations, these lists need to absolutely prohibit outside publication.

Many mailing lists deal with this in a contract, and treat it as a copyright problem — by providing language in the mailing list subscriber agreement that requires list members to not republish mailing list posts outside of the list.