For some time now there have been issues with Blind Carbon Copy appearing in support forum where the mail provider is outlook.com, in searching for a solution it bacame obvious that the issue encompased all mail clients wsending mail with SMTP, indicated it was a bug, or change of policy at outlook.com.
The effect of this issue was that mail send using BCC was not delivered by outlook.com. It appeared that BCC addresses were simply stripped and dropped from the email. The mail would be delivered to anyone in a CC or To field, but BCC recipients simply did not receive mail. At one point I had contact with folk who were able to get some BCC mail addresses to work, but not reliably and they were doing nothing different to others. This reinforced my view that indeed there was a bug with outlook.com
I have had a support request with the Outlook folks for some time on this matter and I have now been notified the yes they do have a bug. Yes, they have a fix and are testing it. Hopefully BCC will soon work again for those using SMTP in the near future.
I will update this post when the fix is released. If I am notified of the release.
Update: All Outlook.com mail accounts should not work with BCC