This site presented me with five lines of text and a button that grants this site permission to read all my email.
How about no?
I'm hesitant to even run it locally and try. I looked through the source briefly and didn't see any dead giveaways, but with how obfuscated, etc. you can make code it almost seems like it'd be safer/faster to re-implement it with help from the code than to try and determine if it does anything tricky.
Does this replicate/clone Gmail's search engine and spam filter? Those are the only things that could differenciate Gmail from other solutions...