First of all you need a real dns name ( not a localhost ), because google com not provide a mail service delivery for localhost or some else domain without normally name, such as mymailserver.com, mail.myserver.com with MX DNS record.

Next, you need check your ip in consist of spammer black list (PBL, XBL). This can check on spamhaus resource.

And the last, You need reconfigure your server for sending and delivery mail.