Separate remittance emails are not receiving with users having outlook email

Our Vendors are complaining they are not receiving the remittance advice emails upon analysis they all have @outlook.com domain. We tried adding these for test purpose and emails are not being delivered. Have anyone faced similar issues with @outlook.com domains and have any resolution. Opened an SR and there was no answer (and their is no blocking of Microsoft email domains from Oracle side as per the analyst).