fra
http://dk2.php.net/manual/en/function.mail.php" The Windows implementation of mail() differs in many ways from the Unix implementation. First, it doesn't use a local binary for composing messages but only operates on direct sockets which means a MTA is needed listening on a network socket (which can either on the localhost or a remote machine). Second, the custom headers like From:, Cc:, Bcc: and Date: are not interpreted by the MTA in the first place, but are parsed by PHP. PHP < 4.3 only supported the Cc: header element (and was case-sensitive). PHP >= 4.3 supports all the mentioned header elements and is no longer case-sensitive."
Tjah, jeg tror nu heller ikke det er mere ...stabilt... Snarere tværtimod, da jeg mener at på de fleste platforme bliver mailen injected i en kø til aflevering - men jeg er ikke 100% sikker ;)
På den anden side springer du så dét led over, og leverer direkte til en SMTP-server som så selv lægger det i kø til aflevering...