We’ve received inquiries from bulk senders who’d like more information on best practices to ensure that their mail is delivered to Gmail users. The way Gmail classifies spam depends heavily on reports from our users. Gmail users can mark and unmark any message as spam, at any time. To increase the inbox delivery rate of your messages, make sure that all recipients on your distribution lists actually want to receive the mail. Visit the following sections for some tips on how to make sure your messages are welcomed by Gmail users.
Authentication & Identification
Subscription
Unsubscribing
Format
Delivery
Third-Party Senders
Affiliate Marketing Programs
Reference from:
https://mail.google.com/support/bin/answer.py?hl=en&answer=81126