You’ll have to read the mail log: /var/log/mail.log
When you send an email from a test account at a Microsoft, it shouldn’t take long, for their mail server to contact your server. If there is no connection for a long time, the problem is on their side, and you’ll have to contact their support.
If there is a contact, but your server rejects it temporarily, then greylisting is activated (on by default). Greylisting stalls not-well-behaving mail servers. That is an important spam filter technique. Greylisting is activated if the UCR variable
mail/postfix/greylisting is enabled.
To enable popular mail providers to bypass that check, see that the UCR variable
/etc/postgrey/whitelist_clients.local and then add their mail servers to it. To catch all their server, use the domain. For Microsoft mail that is