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. Thus, Gmail's filtering scheme acts on domains that are not obviously connected to Gmail.
Delivering to Gmail
Gmail has fairly liberal mail acceptance policies. They do not use any SMTP level blocking or filtering and accept all mail that is sent to them. 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.
Important links and references
Gmail does not have postmaster pages as other ISPs do. They do provide some information for senders at their bulk sender guidelines page.
Gmail does not provide a feedback loop for senders. Subscribers have the ability to single-click unsubscribe from mail using the "show options" menu in the right corner of the message. They are also providing users the option to unsubscribe when they use the "report spam" button.
Senders who wish to take advantage of the Gmail unsubscribe option must meet the following criteria
- The mail is authenticated using SPF, DomainKeys or DKIM
- The sender has a good reputation
- The email has a mailto: option in the List-Unsubscribe header as defined in RFC 2369
Gmail does not offer whitelisting to senders.
Gmail is checking DomainKeys and DKIM for incoming mail, but they are not using any commercial certification services for delivery decisions. Google does own [Postini] and there is speculation that some of the Postini filtering is used in by Gmail, but there is no evidence this is the case.
Support for Senders
Gmail provides minimal support for senders. They do have 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.
Information provided by http://wiki.wordtothewise.com/Gmail