PHP's mail(): proper way to send a 'From' header

**Apologies if this posts twice. I got some crazy response from the server after sending this the first time.** I have a site I set up for a client that has a form their clients can fill out to submit some data. When the form is submitted, I have PHP gather the data and create the body of an email which is then sent to both the owners of the site and back to the person who submitted the data. Because the server hosts multiple sites, I am sending an additional 'From' header so the email doesn't appear to come from the hostname of the server itself (nobody@web.mysite.com). As per PHP's documentation of the mail() function, I am sending the header like so: "From: sender@othersite.com\r\n" I am getting bounce emails from certain ISPs (AOL, Roadrunner, some local ISPs) saying the sender's domain does not exist. It seems that either mails are coming from my hostname (nobody@web.mysite.com), or those ISPs are reading the additional headers incorrectly. Unfortunately, this is not acceptable. People aren't getting their emails, and the hammer is coming down on me. Because I did not have a DNS entry for my hostname, the 'domain does not exist' error I'm seeing in the bounce emails is correct. I do not wish to keep a DNS entry for it (I have added one as a temporary fix), as that doesn't fix the 'From' header issue to begin with, so I would appreciate it if you did not make that suggestion. As far as I know (based on the lack of bounce emails), this worked fine on PHP4, but with our new webserver (running PHP5), I'm experiencing problems. Far as I can tell, the mail() function has not changed between versions. I'm stumped here and need to get this fixed asap. I've tried 'From' and 'FROM', tried a 'Name Here <name@isp.com>' format, and tried terminating with double newlines with and without the carriage return. Nothing seems to work. I've even gone so far as to edit php.ini with a default from address, but that doesn't appear to have fixed anything either. Please help. Thanks in advance, Jay