Hello @Katerina1 ,
There are multiple reasons why a newsletter land in spam
1) HTML not following best practices like the text to image ratio.
2) Spam Triggered words
3) IP address reputation isn't good
4) Engagement is very low in the newsletters.
5) No Opt-out link
You can refer to this documentation to improve the deliverability of your campaigns
https://www.adobe.com/content/dam/www/us/en/offer/pdf/Adobe-Campaign-Deliverability-Best-Practice-Gu...
There are many other factors