Mac:
Here is an example that might help you:
mail("you@hotmail.com", "Your Registration", "<p><b>Thank you for registering.</b></p>",
"From: MortgageGuardian.com<support@mortgageguardian.com>\r\nContent-type: text/html" );
The last part in " " is the header portion. Here you can set your from address as I did above.
The ending: \r\nContent-type: text/html allows me to use HTML tags in my email body (the third set of " ") and if the email reader of the receipent is HTML compliant, the tags will work. That way I can make text bold, certain colors, even include entire tables.
Good Luck.
Bruce