Gmail is a webmail service provided by Google. Gmail is one of the largest mailbox providers with millions of individual users. There are also a significant number of individuals and small businesses that host their entire domain at Google through Google Apps. Thus, Gmail’s filtering scheme acts on domains that are not obviously connected to Gmail.
There are differences between the Google Apps filtering and the Gmail filtering, but they do seem to share some similar features. Here we’re primarily talking about mail to gmail.com addresses rather than Google Apps hosted products.
Gmail has fairly liberal mail acceptance policies. They do use some SMTP level filtering, but an IP has to have an extremely poor reputation to be blocked at Gmail. Gmail uses custom content filters to process mail after receipt and place suspect mail into the spam folder. Gmail does not publish and does not appear to enforce either connection or sending limits.
Google recently published a postmaster page that includes a Spam Rate Dashboard, Domain and IP Reputation Dashboard, Authentication Dashboard, Encryption Dashboard, and Delivery Errors Dashboard. They also provide some information for senders at their bulk senders guidelines page.
Gmail does not provide a traditional feedback loop for senders. During the San Francisco M3AAWG meeting in February 2014, Gmail did announce they were opening up a pilot FBL to M3AAWG Email Service Providers1. Gmail is the first ISP to offer unsubscribe links in the mail interface2.
Senders who wish to take advantage of the Gmail unsubscribe option must meet the following criteria
Gmail does not offer whitelisting to senders.
Gmail does check DMARC authentication on inbound email.
Gmail is checking DKIM for incoming mail, but they are not using any commercial certification services for delivery decisions.
Google provides a postmaster website, and a self directed troubleshooting webpage. If that troubleshooting does not work, there do provide a contact form. This contact form is used to update their filters, but Gmail will not respond to any inquiries sent.
Gmail does not use any known commercial spamfilters.