Who is your email provider? I was reading about this in the newsgroup, and there a hotmail user was having the same problem as you, but I was not able to replicate his issue with my own hotmail account. But he is in the USA and I'm in the UK and we are almost certainly routed via different servers.

The handing of addresses and bcc is really the responsibility of the smtp server and in principle the email client has little or no influence over the outcome.

How did you learn of what happened? Did one of your correspondents report it to you? What you see in your own Sent folder isn't indicative of what is seen by your correspondents.

It would be interesting to know if reverting to the earlier version of Thunderbird changes it back to working as you expect.

In another thread, I read this could happen if you leave TO completely blank. Always put yourself in the TO slot when the real recipients are in BCC.

Also, there was a problem with Outlook/Live/Hotmail accounts and BCC's recently: the BCC's never got the message.

I asked a secretary for a group to install Mozilla and only send mail using Bcc so everyone's email address is not broadcast to the entire group. I received an email from him and I questioned him why he did not use BCC. He replied he did use BCC. I setup a Teamviewer connection to his desktop and sent an email to my earthlink email address and my embarqmail address. When I opened my mail it listed both email addresses as if it was sent using the To: address line. I noted he had 45.2.0. I tested the same process using my Mozilla (44.?.?) with different email addresses back to the other desktop and it did show only one BCC address. My Mozilla had a message that an upgrade was ready. I did the upgrade to 45.2.0 and retested. The BCC function now fails on both our computers. I believe I have proved there is a problem with the upgraded software. I can re-install the older software on my pc, but I know BCC was working before I upgraded to 45.2.0.

@Jscher2000 I tested as you did (TB 45.2.0)

1. To: myself and two bcc. Result: To myself in every mail.
2. To: empty and two bcc. Result To: empty in every mail (undisclosed-recipients in header)

It never revealed the two bcc

Maybe you should look at the smtp-server-provider.

Today I sent an email to an address list that included one of my email address using earthlink.net I will test other email providers to determine what my problem is/was. Thanks for everyone's help

I can safely surmise that the problem is with Thunderbird 45.2.0 and NOT with the Server. I sent similar BCC emails using the Windows 10 Mail program and everything was received in each account privately. Is this usual, that fixing one issue creates another?