Do you see this error in your mail.log?
“Please contact your Internet service provider since part of their network is on our block list (AS3140)”
Go to this website to request Outlook team to remove your IP from their blocked list.
https://support.microsoft.com/en-us/getsupport?oaspworkflow=start_1.0.0.0&wfname=capsub&productkey=edfsmsbl3&locale=en-us&ccsid=635611717755428181