In fact G-mail is exceptionell in detecting many things, and in case of doubt throws it into spam, together with a note from G-mail in red saying this may not be from who it says it is!
I still check it out, but it is a good starting point. We all know Bill Gates and Co., don't often give anything away - so forget it!
Maybe I'm actually a millionaire 100 times over - just don't know it! From PayPal - I've had dozens, and I've signalled them direct to PayPal, but never had a message from them saying thank you - or anything else!