If you’re setting up a new server it’s possible it’s been given an IP address that’s on a spam blacklist due to spamming in the past. They will block you because they suspect that spamming will continue, despite the address being owned by a different person. Or there could be a bunch of other reasons why they might not like you or your site.
You could try Mailjet - https://www.mailjet.com/ They work OK from Discourse for me, although depending on your email volumes you might find other providers who are cheaper.