We are currently unable to deliver mail to domains protected by Microsoft's olc.protection.outlook.com mail servers. This is due to a certificate problem on Microsoft's end. Affected domains include hotmail.com, live.com, outlook.com as well as many other third-party domains hosted under their service.

Update 1: We have applied a workaround on our end to allow mail delivery to these domains to work. Any stuck mails due to this issue should now be delivered.

Update 2: Our mail queue is now clear of stuck mails connected with this issue.

Update 3: A contact at Microsoft has acknowledged the problem and indicated they are working on it.

Update 4: Even though the issue still persists at Microsoft's end, we will close this incident as the workaround is enabling mail delivery to function as normal. We will remove the workaround once we have confirmed that Microsoft have resolved the root cause.