My mails to gmail are going to Spam

My mails to gmail are going to Spam

Till a couple of days back it was working perfectly.
But all of a sudden, the emails sent to GMAIL inboxes are being treated as SPAM.

GMail states :
Why is this message in Spam? It's similar to messages that were detected by our spam filters.   Learn more

Can you please help us on this, as this is seriously affecting our end users since we are not able to update them about their registration details, since most of our users are in Gmail.