An earlier COM version required that a letter has at least one "To"
receiver. Therefore WinCOM 30 changes the "Bcc" receivers to "To" receivers
if there are no "To" receivers.
Now this restriction is no longer there and newer WinCOM versions no longer
change "Bcc" to "To".
So you can either install WinCOM 36. Or you can keep WinCOM 30 but then you
must add yourself as a "To" receiver if all other receivers are "Bcc".

BTW. If you had written in your first text what WinCOM version you are
using, I could have immediately given this answer.