Email BCC header confusion

I am very confused about what htis is for. I am using Pear Mail for SMTP (at the moment) on a Windows server for testing, then moving to a Unix server for real use.

I need to send to a list of recipients, none of whom must see the other addresses - ie BCC.

As a test, if I send an email to four recipients, and also put in a BCC header with the four addresses, all emails arrive, but each can look at the message header and see the full BCC list - which slightly defeats the point of a BLIND carbon copy. So, I leave out the BCC header completely, and all is well. (I'm told there is a Windows Server issue about stripping BCC headers).

Presumably, when I move to using a Unix mail factory, I can just leave the BCC header in and all will be well?

But - what is the point of the BCC header? It seems to say "here is a list of addresses you must not show to anyone". So why put it in at all, what is it for?

Since it sounds like you are sending this To: each recipient and you are putting in a BCC header, it is possible that you are forming the BCC header incorrectly and it is not being processed as a BCC and is simply being include in with all the rest of the data. Perhaps post your actual code, xxxxxxx out any sensitive information...

I think I may have sorted it. It seems that the SMTP and Unix mail factories operate quite differently. (Or - possibly, see below).

With SMTP, all recipients must be listed as param 1 in send, and the contents of the To/Cc/Bcc headers have no effect on what gets sent - they are just receiver information. Added to that, a Windows server does NOT strip out the BCC header for some reason, as stated above. So, with SMTP, BCC is effectively useless.

BUT, with Unix mail, it will build a recipient list from the union of the To/Cc/Bcc headers, and if you also put those addresses in send param 1, everything gets sent twice.

It is possible however that the reason is not the difference between SMTP vs Unix mail support, but is due to the difference between Windows servers and Unix servers. I cannot tell, because my Windows test server has to use SMTP and my real internet server is Linux and does not use SMTP, so I cannot try it the other way round.