* website hosted on Win2000, IIS 5.0 (closed intranet, so I can't link you)
* mail server on FreeBSD using Postfix, on the same network
* php.ini configured with default UNIX setting (sendmail_path = sendmail -t -i)
* mail() function returns true, but no mail is sent
The other tech here doesn't know PHP but does not understand how it is possible to send mail from another machine on the mail server without some kind of login information. But I cannot find out if this is actually necessary, or how to implement it if it is.
If your ISP allows outgoing SMTP connections, you could probably set up your own mail server.
If it doesn't, then they'll have a special domain you send all SMTP emails to (like for Optonline, it's mail.optonline.net ). This is all configured in PHP.INI