What is your domain name? We can test with that.
It would also be helpful if you were to post the HEADER portion of the rejected e-mail from Gmail. It probably has a code which begins with a 4 or 5 and has some other numbers following it, along with some other information about the rejection.
To go back to your question, rejection can happen for many reasons, including:
bad reputation (being on a spam database)
no SMTP authentication
sending to a bad e-mail address
there are lots of reasons e-mail can be rejected.
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