Postmaster Tools
	
		If you send a large volume of emails to Gmail users, you can use Postmaster Tools to see: 
	 
	
			If users are marking your emails as spam
		
		
			Whether you’re following Gmail's best practices
		
		
			Why your emails might not be delivered
		
		
			If your emails are being sent securely
		
	
	https://support.google.com/mail/answer/6227174
 
	https://postmaster.google.com