Hi E27,
I have much the same problem you describe. Until about one week ago, I could look at any Sent item from any account and see both BCC and CC in the message header, with the complete list of addresses I had sent to.
In Mail 3.6 my header preference has always been "Default." In toggling Long Header repeatedly, never once has CC or BCC appeared in the header, though many other header items have. BTW, I rebuilt the mailboxes, reindexed the messages, and repaired disk permissions (per Mail Help). No fix.
In my sent boxes are items from two current POP accounts, one POP account no longer in use, and a GMail IMAP account. The Sent POP messages are resident on my Mac, and none display the BCC or CC info they always have readily in the past. So the vagaries of GMail and IMAP aside, why no CC or BCC info?
I share the concern that this is something whacked in Mail 3.6 or a supporting file. It seems the "Long Header" toggle and/or Mail Preferences aren't functioning predictably. Has anyone else encountered this? Found a fix?
Thanks for listening.