Fixed - Microsoft has lifted the block and their servers are now accepting mail from the US SMTP. (01:47 UTC — Oct 2)
At this time messages sent from the US SMTP server to Microsoft email domains like hotmail.com, live.com, outlook.com, and their international variants are being blocked by Microsoft.
The cause of the block has been identified and corrected and we are now waiting for Microsoft to remove the block.
We’ll update this post as soon as the block has been lifted or when there is other information to report. (20:17 UTC — Oct 1)