Gianni's right. That happens to a lot of people, so you have nothing to worry about. You could quickly fix that by reading this article on
http://folderly.com/blog, which helped me a lot. I also learned a few valuable things about email deliverability, which is essential. All you need to know is that you should categorize your mailing list, stay relevant to the timeline, and customize your subject lines. Recipients get suspicious even before they open a generic email. A boring subject line such as "Email for [Name]" or "Message from [CompanyName]" looks shifty enough.