If you a mailer you should be doing the following to help ensure your deliverability.
1) Know the domain makeup of your list.
2) Create / acquire seed accounts of the domains that have the largest percentage of users.
3) Test your messages (inbox placement) with your seeds before you send.
4) If your having a spam folder problem...test (see below) and isolate what the problem is and fix.
A. Back-end set up: spf, dkim, rdns, etc...
B. IP / domain reputation: any IP / domain associated with the mailing
C. Content: Everything in the message body / subject line, including template
At a minimum you can set up seeds for aol, gmail, yahoo, hotmail/outlook, as they can account for as much as 85%+ of a list (including parent domains using the same mx). Then set up / acquire any other seed accounts possible. For Gi, you could set up an account with spam assassin and pyzor/razor add-ons.