Let's be very clear about this for consumer facing campaigns there are 4 major ISP's that manage the majority of consumer inboxes.

?MSN/Hotmail

?Yahoo Mail

?AIM Mail

?Gmail

What Does this Mean?
Basically unless each of these mail platforms relays your message to the primary folder, your email campaign is far from optimized.

What Causes mail to be delivered to a bulk/spam Folder?
All of these ISP's allow their users to report spam with a ?report spam? button. The ISP uses this feedback to create a profile for your mail. If users are reporting your mail as spam you will run into problems.

What Can I do to make sure I do not create ISP Spam complaints?
AOL recommends keeping spam complaints below 1-3 percent of traffic, depending on volume. This figure is unique to AOL's user base; it's too generous when applied as a general standard. Be at or below the range of one complaint per 6,000 to 8,000 messages, or 0.013 percent.
Minimize Complaints
Minimizing complaints always starts with practices used to collect e-mail addresses. It should be obvious by now sending unsolicited e-mail only gets you in trouble. Mailing lists with the lowest complaint rates are either confirmed opt-in or properly managed single opt-in. If you have a solid permission-based list but still find incoming complaints are higher than the optimal rate or are rising, consider the following:
?Brand your subject lines. Mail systems with spam complaint buttons offer it at the inbox level. A recipient need only to scan subject lines and decide which messages not to delete immediately. A subject line such as "Exciting offers for you, Bob!" will surely be marked as spam. Consider using your company or newsletter name in brackets at the beginning of your subject lines.
?Consider including unsubscribe instructions at the top of your e-mail, in addition to the footer. Some users use the "report spam" button as an unsubscribe method and won't scroll through an entire message to find that link.
?Include instructions for users to whitelist your domain. This prevents a user-based filter from mistaking your message for spam and either diverting it to the spam folder or prefixing "[SPAM]" to the subject of the message.
?Provide a preference update page. Disclose how your organization will use a subscriber's e-mail addresses, and how often. Allow subscribers to select preferences on the opt-in form, and link from e-mail to a preference or profile update page.
?Avoid spammy looking content. Try not to use garish, bold fonts; large, red letters, and the like. Avoid images with poor compression quality. A clean, readable design isn't as likely to be mistaken for spam.
?Don't over e-mail. If recipients expect to receive a few informational e-mail messages each month from your company, don't suddenly start sending two or three each week.
?Don't send unexpected e-mail. If subscribers opted in to receive your "Trends and Tips" newsletter, don't send them your hard-sell e-commerce messages, unless they clearly requested them.
?Include opt-in information. If possible, add to your e-mail admin area information, such as the subscriber's e-mail address, date of opt-in, and how she potentially subscribed (product registration, white paper download form, sweepstakes entry, etc). With many subscribers receiving dozens of commercial e-mail messages daily, it's easy to forget signing up for your newsletter -- and then to file a complaint.

What Can I do to test my ISP deliverability?
We recommend you use a service like EmailReach. Their trial is free and let's you know where you stand in about 5 minutes.
http://www.emailreach.com/default.aspx

Following these guidelines should help you to avoid being bulk foldered by the main ISP's.