[rake]

Are you talking about vbulletin or another script? vBulletin sends mail marked as being from the e-mail address set in your cp options.
For other scripts, the syntax is:

mail ( string to, string subject, string message [, string additional_headers [, string additional_parameters]])

To the change the from header, you'd have to use something like:
mail("someone@x.com", "subject", "message", "From: me@domain.com");