This could be happening because of a few different issues:

1.) Your ISP blocks outbound port 25. This is increasingly popular on home ISPs to stop spam. They will force you to use their central mailserver.

2.) You are inadvertently blocking outbound port 25. This would be something you've actively done. Check your ufw/iptables rules for spurious deny rules.

3.) The remote services are blocking you.

Double check your mail log with sudo less /var/log/mail.log and make sure you're not missing any obvious issues.

You can also debug more simple network issues with simple tools such as ping. For example:

ping mx1.hotmail.com

If ping fails, there's a serious network-level issue. If you can't ping it that might suggest you're being blocked completely.

You can also use Telnet to test. For example:

telnet mx1.hotmail.com 25

That command should open up a raw connection to the remote mailserver. If that doesn't work, you know it's not Postfix that's causing the problem. It's a network issue instead.