If you are not spamming it should be go to directly to inbox. Gmail has very smart filters, I guess they do check on how many similar emails are there, minor differences like adding random numbers or text will confirm it as spam.
Other things you should look at if your domain or any IP's are blacklisted.
If you have registered users for specific email subscription; Have then add to white list. Else if you are spamming on random emails, then it will be reported and your product and company can also get banned.