This is a common issue and can have many reasons such as:
1. Your DNS records are not properly configured
2. Your sending IP has a bad reputation
3. Your domain has a bad reputation
4. Your recipients marked your emails as spam
5. You're sending suspicious attachments
6. …
The best way to know why your sent email has been delivered to the recipient as a spam, is to check mail testing tools such as mail-tester.com. This website let you know why your sent email has been classified as spam and various tips on how to solve the potential problems. You should do your best by making sure that you get a 10 out of 10 score from mail-tester.com