Does this happen for multiple users with Gmail and Hotmail, or just one? I
have never had Gmail send any of my wiki-related notices to the spam
folder. It looks like your domain actually has SPF set up, from the headers
you included, so if it isn't an issue with individual users, it could indeed
be a blacklisting issue. I checked your domain's IP on Spamhaus, though,
and it said it wasn't blacklisted.