Problem with BCC with Desktop apps using Microsoft email accounts

[John A]

If are using a desktop mail client like Windows Live Mail, Thunderbird, eM Client, etc.; with many @hotmail, @outlook, @live, @msm accounts, there has been a problem with BCC for a few weeks at Microsoft's end. This issue does not occur if you use a web browser or the Windows 8/10 Mail app:

I have been having this BCC issue with my Hotmail accounts in either Thunderbird or eM Client: 

Case 1: if you send to BCC and have no one in the To field, Microsoft servers move all the BCC addresses to To so everyone can see all the addresses!

Case 2: If you have one or more addresses in To and some in BCC, the BCC addresses are deleted by the Microsoft servers and are never received!

http://answers.microsoft.com/en-us/outlook_com/forum/opeople-ocontacts/outlookcom-moving-addresses-from-bcc-field-to-the/b4505d03-7b66-4f98-a878-f1d82fcf42a6 

https://forum.emclient.com/emclient/topics/bcc-functionality-has-stopped-working 

There is no indication from Microsoft whether/when this basic email functionality will be fixed.