Tested with PHP 4.2.2 on Linux: Please note that if you're using Unicode (mb_language("uni")) and you attempt to send mail with mb_send_mail(), you will need to base64_encode() the message body - mb_send_mail() doesn't do that for you. It does, however, issue the correct message headers, so you don't need to worry about that.
Also note that neither mb_language() nor mb_send_mail() is able to convert your message to UTF-8. *You* need to provide the UTF-8-encoded (and base64-encoded) message, and then mb_send_mail() will issue the correct message headers.
Here's an example of sending an UTF-8-encoded message with mb_send_mail():
mb_language("uni");
$body = chunk_split(base64_encode("International characters"));
mb_send_mail("[email protected]", "Subject", $body);
If the receiving mail client supports UTF-8 properly, you will be able to send messages that contain a mix of all kinds of characters (e.g., you could send Thai, Chinese, and Danish characters in the same message). Not all mail clients support UTF-8, though. At the time of writing, some of the more popular Windows email clients - Eudora and Pegasus Mail - don't. There are a number of email clients with working support for UTF-8. These include Outlook Express, KMail, Mozilla, Netscape 6/7, Sylpheed, Evolution and others.