Our member, Sasha from Rufus and Roxy has an excellent answer to this very important question!
There's lots of reasons your emails could be going to spam. Generally you want you emails to be plain text, no more than 2 links, limit formatting (like bold, underline etc) and limit photos. You want it to seem more link an email from a friend/colleague, than a company.
There are tools to help with this, too!
Mail Tester will tell you if your copy is spammy.
And, Playground from Zurb shows you what your subject will look like on mobile.