I'm reading Postnotices to groups, nice to hear about this thread also. Very nice work btw
However somehow I prefer the idea of sending notifs to a division of members (groups) more than going after division of forum/category, and how it is sent only on purpose by the poster rather than automatically sends everything. By that way only important infos are delivered, so each time I receive an email I'm sure there is an important announcement within
Still you're right about it being closer to Google Groups

It's great that it can displays the post content within the mail just like GG, if only it works seamlessly like GG and Gmail do with each other (automatically logins I mean) then it's GG all the way
I'm not suggesting you to develop plugin that delivers PHPBB mails that can auto login you to the forum upon clicking though, since it poses quite a lot of security problems

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Still it is wonderful if it comes true