This happens to many people, that's why sometimes it is better to manage them with mailchimp or ymlp.
The newsletter goes to spam for many reasons, for example:
1. (IP) Server with bad reputation
2. The newsletter has attachments
3. Some words are blacklisted, I think they also recognize if the title is ALL IN CAPITAL LETTERS
4. Heavy template design (lot of images, linked to unreliable server).
5. Email of the sender different from the email of the website, for example, if the sender is:
[email protected] and the website is sitename.com, I think that it would be better to have it sent from
[email protected]