Outlook.com Moving Addresses From Bcc Field to the TO Field When Email is Sent via Desktop Email Client

I am using and have been using "Postbox", a desktop email client, to send and receive all my emails through my Live account on "Outlook.com" using IMAP. Everything has worked perfectly between "Postbox" and "Outlook.com" for several years and I haven't changed anything on my end. About two weeks ago, I started having problems. Now, when I put a single name or a group list into the Bcc field on my "Postbox" desktop email client and send that email through my outgoing SMTP server at "Outlook.com", the email goes through "Outlook.com" and "Outlook.com" moves all the names from the Bcc field to the TO field. I am assuming this has something to do with the new changes associated with the updated "Outlook.com/Outlook Mail" web mail process.

Notes:

1. When I do the same thing using "Gmail" for my outgoing SMTP server, my names stay in the Bcc field as they should.
2. My Outlook.com web page now shows Outlook Mail in the top left corner.
3. I am using imap-mail.outlook.com Port 993 and smtp-mail.outlook.com Port 587 for my servers. I tried switching to imap-mail.live.com and smtp-mail.live.com as one MS web site suggested regarding the new changes to Outlook Mail but those new setting would not connect.
4. I tried using Mozilla Thunderbird instead of Postbox as my desktop email client and the same problem occurred in that the name in my outgoing Bcc field was put into the To field after the email was processed through Outlook.com.
5. Based on all of the above, I believe the problem has something to do with the way the new Outlook Mail is trying to process names from the Bcc field in emails sent from my desktop email clients.

I hope someone from MS can look into this problem and get back to us.