As for the next post that says that the idea is to get them to came to the board: if what you sent them (even if it is the entire thread) is interesting they will come anyway, to add to the discussion or because they know now that your site hosts interesting discussions... i don't believe in the idea of "I'll show you something but you have to click for more" (I don't believe that it works as expected, that is).
My only concern about mailing the entire therad is the intimidating potential size of the mail. Another reason for doing it on a post by post basis is the fact that what gets your attention is ussually one post, even in the context of a discussion (but maybe that's just me...I always wished I could send posts rather than threads as a user). I also would be somewhat lazy if I got a huge email...even my friend does not know if that is actually interesting for me so I would like him/her to show me something interesting and let me decide if it is worth reading more...
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