Some of the best practices which increase the probability of your email landing into user's inbox and not in spam are following:
Don't use personal address: Instead of using your personal email address you should always use a paid one provided by your organisation / company.
Check links: Spam filters maintain a close watch on number of links as well as reputation of the domain to which you are linking. So a mail having 5-6 links is more likely to be treated as spammy in comparison to a mail having only 1 link.
Email Subject: Having a personalised email subject helps in increasing the probability of email landing in inbox. Don't use all capitalized subject line since it is known to trigger spam filters. Similarly words likedear,free,% offandreminderare known to trigger spam filters. For more detailed coverage on this issue referthis e-consultancy article
Email Content: Having too many images is known to trigger spam filters. Similarly not havingunsubscribe linkor having a verysales pitchemail will also trigger spam filter alerts. Having an email with very differenthtmlandplain textversion may be another reason. As a general practice write content with not many images or links, ensure that html has been properly coded, use personalization and write as if you are talking to a friend.