This is a frequently asked question, both here and around the internet, a quick search of this forum or most anywhere will find pointers to the basics. If you have specific questions, follow up with the details.
I'd guess most anything related to it, eg. I copied 'mail server google was blocking my emails' from your original question and searched with that, it found quite a few threads that look promising. Or try a google search for 'gmail.com blocks email from my server'.
