All of the Microsoft owned email addresses (@outlook.com @hotmail.com @live.com and @msn.com) are notoriously strict at filtering out emails and blocking sender IPs, especially those coming from large shared IP addresses from email providers like Sendgrid (or other large email API services)…
we’ve had issues over the years sending emails to those addresses, even from some of the largest and most well known email services and CRMs…
The only sure-fire way to get round the problem and maximise deliverability is to use your own IP address (or addresses) for sending your emails…
Alternatively, many people seem to have solved the issue using smaller email API service providers (like postmark or send-in blue) which tend to have less users on a single IP address, therefore the likelihood of sharing an IP with someone sending problematic emails is lower, so the IP address is less likely to get blocked by the Microsoft email accounts… They also seem to update and change their shared IPs (and remove blocked ones) more often than some of the larger companies, which might also explain the higher deliverability compared to services like Sendgrid…