I'm using php mail() function to power a mailing list. instead of using mail in a loop i'am thinking about sending all emails through Bcc header, does it improves performance or is the same thing (or worst)?
Problem is everyone on the list will see everyones emails - a big no-no. Check out scripts like phpList for more powerful solutions. In my opinion, you should get an external mailing list provider, such as aWeber.
No, with Bcc header they dont, only with Cc (Bcc means blind carbon copy) Bcc is the private version of Cc