If you send a large volume of emails to Gmail users, you can use Postmaster Tools to see:
Troubleshoot delivery issues with Postmaster Tools
Gmail Postmaster Tools provides senders with metrics on parameters such as reputation, spam rate, feedback loop, etc. It can help you check compliance to the Gmail bulk sender guidelines, as well as identify issues that can cause trouble with the spam filter.
Authentication & Identification
Why is it important to authenticate your messages?
Authentication ensures that your messages can be correctly classified. Emails that lack authentication are likely to be rejected or placed in the spam folder, given the high likelihood that they are forged messages used for phishing scams.
In addition, unauthenticated emails with attachments may be outrightly rejected, for security reasons.
To ensure that Gmail can identify you:
- Use a consistent IP address to send bulk mail.
- Keep valid reverse DNS records for the IP address(es) from which you send mail, pointing to your domain.
- Use the same address in the 'From:' header on every bulk mail you send.
We also recommend the following:
- Sign messages with DKIM. We do not authenticate messages signed with keys using fewer than 1024 bits.
- Publish an SPF record.
- Publish a DMARC policy.
Learn more about email authentication.
https://support.google.com/mail/answer/6227174 - Postmaster Tools