To find out why your email is being marked as spam, you can use www.mail-tester.com:
1. Copy the email address you see here.
2. Go to the design of your campaign that is being marked as spam and click on the cogwheel.
3. Paste the email address you copied from mail-tester.com into "test your campaign" and click "send test".
4. Go back to mail-tester.com and click on "check your score".
5. You will then see the score and improvement points, click on the arrow next to the improvement point to get more information. Adjust these points to avoid being marked as spam in the future.
You can use mail-tester.com for free with a maximum of three tests per day per IP address.