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.
Here at Gmail, we work very hard to fight spam. While in some cases we may classify a message as spam and deliver it to the spam folder, we also try to find ways to reduce the amount of spam being sent to Gmail in general. If we detect that a message has a strong likelihood of being spam, we’ll block the message from being sent to Gmail.
A message might be blocked if it contains suspicious-looking or spammy text or if the sending IP has had a history of sending unsolicited messages.
Is all of the mail I’m sending being blocked?
It’s likely that only a subset of the messages which have a strong likelihood of being spam are being blocked and not all of your messages. However, to help improve your deliverability, we recommend reviewing our Bulk Sender Guidelines.
If you’re forwarding mail to Gmail and your domain also forwards spam, we recommend reviewing our mail forwarding best practices.