Most common reasons are due to Google blocking your mail server IP due to the unsolicited mail originating from your IP address. This could be caused by too many abuse complaints and/or too many inactive recipients in your databases. As per the Google guidelines “if you are now in compliance, note that it may take some time for delivery to improve” As such we recommend you to enable DomainKeys (DKIM), rDNS(ptr records) and SPF records, they are becoming a common demand among email providers, mainly Yahoo, gmail and Hotmail.
These two methods of email authentication designed to verify email integrity, by linking a sender to your server or hostname. SPF (Sender Policy Framework) is a feature that allows a recipient server to verify that an email message has really been sent from the domain specified in the From: field. DKIM helps verify the sender and integrity of a message.