http://slashdot.org/articles/04/06/21/1150236.shtml?tid=126&tid=217&tid=95 Hotmail is one of the big culprits who completely block messages from Gmail. Yahoo Mail sends it to the Junk Mail folder. I had a friend who has an account who sent me an email to my hotmail account and I never recieved it. It wasn't an invite because he already used one up.
I've had no problems sending from my Gmail account to other peopel with Hotmail....I jsut sent one to my own hotmail account to test it and it worked okay.
