What is the way to overcome email spam issue with Microsoft 365?

I am Ram working in an IT firm. We recently moved from Rackspace to Microsoft office 365. We do face issues such as Low response rate, Emails sent and emails received are into spam while doing email marketing. Is there any specific fix for this? Please help. I have made alterations such as removing multiple links from email body, reducing punctuations and reduced content. We still face the issue. this was never happening with Rackspace though.