Gmail Postmaster Tools provides senders with metrics on parameters such as reputation, spam rate, feedback loop, etc. It can help you prevent your emails from being blocked or sent to spam by Gmail.
In order to prevent spam, Gmail refuses mail from IP addresses that are not authorized to send mail. The determination of whether or not an IP address is authorized to send mail is made by the ISP that provides you with the IP address. This list typically contains consumer IP ranges offered for dialup, DSL, or other broadband access.
What can I do to fix this?
Your ISP may provide SMTP relays which will accept mail from your IP, and these servers should be authorized to send mail. Some ISPs may provide a way to get an IP that is authorized, either by upgrading to business class service or static IP service. You should reach out to your ISP to see what options are available.
Another alternative is to send mail through your own domain’s servers, either by configuring them to allow relay from your IP address, or by using MSA (mail submission agent). Learn how to use Gmail to send mail from a different address.
Please note that we are unable to whitelist IP addresses or otherwise make exceptions.