The BCC addresses specify the recipients that get a copy of the message, but recipients in TO and CC are not supposed to see the addresses of BCC recipients. This means that if you are one of the TO or CC recipients, you are not supposed to be able to determine the BCC addresses and they should in fact not even be included in your copy of the message.
Interestingly, it's unclear whether BCC recipients are supposed to be able to see each other addresses, as explained in great detail in the Wikipedia article on BCC. When sending e-mail, Rebex SMTP and most other cliens use the first case:
When a message containing a "BCC:" field is prepared to be sent, the
"BCC:" line is removed even though all of the recipients (including
those specified in the "BCC:" field) are sent a copy of the message.
Which means that none of the TO, CC or BCC recipients will be able to see the addresses in BCC header. In this case, even Gmail won't be able to see the BCC addresses.
However, some other e-mail clients prefer the second case approach:
In the second case, recipients specified in the "To:" and "CC:" lines
each are sent a copy of the message with the "BCC:" line removed as
above, but the recipients on the "BCC:" line get a separate copy of
the message containing a "BCC:" line. (When there are multiple
recipient addresses in the "BCC:" field, some implementations actually
send a separate copy of the message to each recipient with a "BCC:"
containing only the address of that particular recipient.)
And this includes Gmail itself, which actually sends a separate copy of the message to each BCC recipient (containg only his address in the BCC field). But this behavior seems to be uncommon (your Yahoo/Hotmail/iCloud observations confirm this), and even Google states that:
Keep in mind that if you're Bcc'd on an email that was sent using
another email program or service, Gmail may not be able to display
this information.
Which is actually a nice summary. This also means that e-mail clients (including Rebex IMAP or Rebex POP3) will only be able to read BCC addresses if the messages in the mailbox actually contain the BCC headers, which (as explained above) is usually not the case.