After you have uncovered that you may have some bulking going on, you will need to get a rough understanding as to what the underlying causes might be before being able to put a corrective action in place. Here are the some of the main things to look at:
IP / Sending Domain Reputations
Reputation of your sending IPs or sending domain’s play a huge part in the decisions behind many spam filtering. Often impacted heavily by engagement, these filters tend to be the most common stumbling block with senders experiencing spam / bulking issues. Separating your mail streams by subdomains and various IP pools will help you monitor and track variations in reputations between them. This is also a great way to keep things like transactional mail for password resets or new account registrations isolated from possible bulking issues.
Message Construction / Headers
Ensure that the headers within your email are correct, incomplete or incorrect headers can cause red flags. Additionally, incorrectly formatted multipart messages or content encoding issues will cause ISPs to give up on trying to deliver your mail to the end recipient. Take your time to correctly format, style and encode your messages to best deliver your mail.
URLs within the email
The links / URLs within your email can also be the cause bulking.
Testing is key
While using some of the example sites that I have listed above may be a good place to start as an overview, these should not be taken as the absolute truth. Testing with known variations in which you know will go to the inbox may help you isolate those items with the most impact. Often, you will see that you can get it down to just a single element that is causing the bulking. Based on those results, you will have a good idea of what changes you will need to make to be back in the inbox!
Other areas you like to test? You can reach me via email – mveilleux [at] dyn.com or Twitter.com/MikeVeilleux.