*Note:* It is worth noting that the mail() function is not suitable for larger volumes of email in a loop. This function opens and closes an SMTP socket for each email, which is not very efficient. For the sending of large amounts of email, see the » PEAR::Mail, and » PEAR::Mail_Queue packages. * Note:* The following RFCs may be useful: » RFC 1896, » RFC 2045, » RFC 2046, » RFC 2047, » RFC 2048, » RFC 2049, and » RFC 2822.
Copy from PHP Manual.