You probably want to sign up for SNDS: https://sendersupport.olc.protection.outlook.com/snds/faq https://sendersupport.olc.protection.outlook.com/snds/faq That

You probably want to sign up for SNDS: https://sendersupport.olc.protection.outlook.com/snds/faq https://sendersupport.olc.protection.outlook.com/snds/faq That should allow you to be more proactive about users reporting your messages as spam, either intentionally or unintentionally. FWIW: We've been sending Microsoft properties e-mails for over a decade, fairly small scale (maybe 5-20K unique recipients at MS properties in a month), and every 2-4 years we have to submit our IP to their "whitelist me" site and then we're golden again. This time was different, when we submitted our IP to the whitelist site it said "Nothing is blocking your ability to send to us". They did end up responding to our whitelist request a week later asking if we were good or still needed help, which is a first.