There are many reasons an email could be marked as SPAM but one of the things is your content -- the words you use, the length, how you insert images, etc.
There are a couple of 3rd party services that you can use to get an idea of how mail providers analyze your emails:
You send emails to one of these services and they reply with a breakdown of all the positive and negative factors of your emails. This helps you isolate and fix specific issues that may be sending your email to the Spam folder rather than the inbox.