RCPT TO: header
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RFC821 4.1.1:
  RECIPIENT (RCPT)
    This command is used to identify an individual recipient 
    of the mail data; multiple recipients are specified by 
    multiple use of this command.
RFC821 2.
    If the SMTP-receiver can accept mail for that recipient 
    it responds with an OK reply; if not, it responds with 
    a reply rejecting that recipient (but not the whole 
    mail transaction).

As it stands right now, mail() in Win32 PHP is sending the
entire list of recipients in a single RCPT TO: header.  It
also does not include the addresses from any additional CC:
or BCC: headers that may be added.

As a result, the success of sending mail to more than one 
recipient depends heavily on the implementation of the MTA
that the mail is sent via.