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:
no rDNS
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.
Bruce Barnes
ChicagoNetTech Inc
brucecnt@comcast.net
Phonr: (773) 491-9019
Phone: (224) 444-0169
E-Mail and DNS Security Specialist
Network Security Specialist
Customer Service Portal: https://portal.chicagonettech.com
Website: https://www.ChicagoNetTech.com
Security Blog: http://networkbastion.blogspot.com/
Web and E-Mail Hosting, E-Mail Security and Consulting