We host our own Exchange Server. We only have trouble sending email to Outlook.com addresses. Here is the error -
mail.protection.outlook.com[x.x.x.x] said: 451 4.7.500 Server busy. Please try again later from [x.x.x.x]. (AS77712200).
We are not on any block list and have submitted our ip address to Outlook.com Deliverability Support. Also joined SNDS and JMRP per their recommendations.