There are easily-avoidable problems that can make your email look like junk mail, the first being sloppy design.
This includes writing in ALL CAPS or random bright colors, using spammy keywords, and creating one giant image and pasting it in, instead of creating real content for people to read.
Broken HTML code can also cause spam filtering issues, which is why you should test, test, and then test again before you send your campaign. Make sure everything works.
Sending too often or not often enough can also be a problem, so respect your list. Think about how much stuff you’d like to receive. If you decide to change your frequency, do it with fair notice to your list, and expect some unsubscribes due to the change.
For even more detailed tips, read the How to Avoid Spam Filters guide by MailChimp.