If you are sending mail through your gmail.com account, I believe there are various issues that can crop up. But also, you can check the email logs on your site to see what it says, and then see if there are messages in the gmail account concerning bounced messages.
I use Mailgun and webhooks to show bounces in Discourse, and it just works and is really cheap and you ought to something similar. 
Is it possible you use a separate outgoing server than GMail? If so, that service will have logs for outgoing messages.