gmail, aol, and hotmail are very strict with their spam rules. Most likely your server doesn't have the correct DKIM and SPF records being sent so that the email can't be authenticated and is getting blocked.
One way to test is to use a service like: https://www.mail-tester.com/
Send an email to them and get the report and it will tell you the likely reasons why it is getting blocked. Using PEAR and authenticated mail will often solve the problems as well.