This question already has an answer here:
My mail server is set to server.mailtuner.com, but when i try sending to a hotmail user, it goes into the spam folder.
Results on mailtester show the following reverse DNS issue: https://www.mail-tester.com/web-1bcbe&refresh=3
Your IP address 126.96.36.199 is associated with the domain mailtuner.com. Nevertheless your message appears to be sent from server.mailtuner.com. You may want to change the host name of your server to mailtuner.com. Here are the tested values for this check: IP: 188.8.131.52 HELO: server.mailtuner.com rDNS: mailtuner.com
I am using a linode VPS server if anyone is familiar with it and could help.