1. Configure SMTP SmartHost in your primary mail server to point to: smtp.antispamcloud.com (port 587/465/25)
SMTP Smarthost: smtp.antispamcloud.com (port 587/465/25)
See the following How-To articles based on your Exchange server type:
2. Configure your SPF record:
SPF Record: v=spf1 include:spf.antispamcloud.com -all