It appears that some mail domains consider the discourse invites spam. In particular gmail.com and msn.com seem to be affected. Funny enough my own gmail account also considers mailing-list type of messages such as google’s own quora.com spam! (go figure)
Anyway, if you are expecting an invite for the discourse site, checking your local spam-folder might be a good idea! For the time being I have enabled self registration on our discourse instance. However staff still have to approve newly registered users.