I run a few servers at home for various things including a little Raspberry Pi running Debian which emails me alerts when certain things happen. I send these to an Outlook.com account or an Office365 account but being a home IP I often get blocked
“(host mx2.hotmail.com[184.108.40.206] said: 550 OU-002 (COL004-MC5F2) Unfortunately, messages from xxx.xxx.xxx.xxx weren’t sent. Please contact your Internet service provider since part of their network is on our block list.”
So the simple solution is to add a relay host to Postfix.