Many VPS providers have an SMTP gateway that you can use to ensure that your email does not get marked as spam. I use transip.eu and never have problems. Prior to setting up my mail server to use their gateway, I ONLY had problems with Microsoft-based receivers such as @outlook.com. So please, don't use Microsoft for anything email related, as they are currently one of the worst offenders in making it hard for people to run their own mail server.